Porous substrates, articles, systems and compositions comprising nanofibers and methods of their use and production

ABSTRACT

Porous and/or curved nanofiber bearing substrate materials are provided having enhanced surface area for a variety of applications including as electrical substrates, semipermeable membranes and barriers, structural lattices for tissue culturing and for composite materials, production of long unbranched nanofibers, and the like. A method of producing nanofibers is disclosed including providing a plurality of microparticles or nanoparticles such as carbon black particles having a catalyst material deposited thereon, and synthesizing a plurality of nanofibers from the catalyst material on the microparticles or nanoparticles. Compositions including carbon black particles having nanowires deposited thereon are further disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation in-part of U.S. patent applicationSer. No. 11/331,445 filed Jan. 11, 2006, which is a continuation-in-partof U.S. patent application Ser. No. 10/941,746, filed Sep. 15, 2004,entitled “POROUS SUBSTRATES, ARTICLES, SYSTEMS AND COMPOSITIONSCOMPRISING NANOFIBERS AND METHODS OF THEIR USE AND PRODUCTION” by Dubrowand Niu, which claims priority to and benefit of provisional U.S. PatentApplication Ser. No. 60/541,463, filed Feb. 2, 2004, the fulldisclosures of which are hereby incorporated by reference in theirentirety for all purposes.

FIELD OF THE INVENTION

The invention relates primarily to the field of nanotechnology. Morespecifically, the invention pertains to nanofibers, including methods ofsynthesizing or stabilizing nanofibers, articles comprising nanofibers,and use of nanofibers in various applications.

BACKGROUND OF THE INVENTION

Nanotechnology has been simultaneously heralded as the nexttechnological evolution that will pave the way for the next societalevolution, and lambasted as merely the latest batch of snake oil peddledby the technically overzealous. Fundamentally, both sides of theargument have a number of valid points to support their position. Forexample, it is absolutely clear that nanomaterials possess very uniqueand highly desirable properties in terms of their chemical, structuraland electrical capabilities. However, it is also clear that, to date,there is very little technology available for integrating nanoscalematerials into the macroscale world in a reasonable commercial fashionand/or for assembling these nanomaterials into more complex systems forthe more complex prospective applications, e.g., nanocomputers,nanoscale machines, etc. A variety of researchers have proposed a numberof different ways to address the integration and assembly questions bywaving their hands and speaking of molecular self assembly,electromagnetic assembly techniques and the like. However, there hasbeen either little published success or little published effort in theseareas.

In certain cases, uses of nanomaterials have been proposed that exploitthe unique and interesting properties of these materials more as a bulkmaterial than as individual elements requiring individual assembly. Forexample, Duan et al., Nature 425:274-278 (September 2003), describes ananowire based transistor for use in large area electronic substrates,e.g., for displays, antennas, etc., that employs a bulk processed,oriented semiconductor nanowire film or layer in place of a rigidsemiconductor wafer. The result is an electronic substrate that performson par with a single crystal wafer substrate, but that is manufacturableusing conventional and less expensive processes that are used in thepoorer performing amorphous semiconductor processes. In accordance withthis technology, the only new process requirement is the ability toprovide a film of nanowires that are substantially oriented along agiven axis. The technology for such orientation has already beendescribed in detail in, e.g., International Patent ApplicationPublications. WO 03/085700, WO 03/085701, and WO 2004/032191, as well asU.S. Pat. No. 7,067,328, (the full disclosures of each of which arehereby incorporated by reference herein, in their entirety for allpurposes) and is readily scalable to manufacturing processes.

In another exemplary case, bulk processed nanocrystals have beendescribed for use as a flexible and efficient active layer forphotoelectric devices. In particular, the ability to provide a quantumconfined semiconductor crystal in a hole conducting matrix (to providetype-II bandgap offset), allows the production of a photoactive layerthat can be exploited either as a photovoltaic device or photoelectricdetector. When disposed in an active composite, these nanomaterials aresimply processed using standard film coating processes that areavailable in the industry. See, e.g., U.S. Pat. No. 6,878,871, andincorporated herein by reference in its entirety for all purposes.

In accordance with the expectation that the near term value ofnanotechnology requires the use of these materials in more of a bulk orbulk-like process, certain aspects of the present invention usenanomaterials not as nanomaterials per se, but as a modification tolarger materials, compositions and articles to yield fundamentally noveland valuable materials compositions and articles.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed, in one aspect, to a novelpresentation of nanomaterials that enables a broader use and applicationof those materials while imparting ease of handling, fabrication, andintegration that is lacking in previously reported nanomaterials. Inparticular, one aspect of the present invention provides a poroussubstrate upon which is attached a plurality of nanofibers. Thenanofibers may be attached to any portion or over the entire overallsurface of the substrate or may be localized primarily or substantiallyupon the interior wall surfaces of the apertures that define the poresthat are disposed through the porous substrate.

The articles of the invention may be employed as filtration media tofilter gas, fluids or the like, or they may be employed as semipermeablebarriers, e.g., breathable moisture barriers for outerwear, bandages, orthe like. The articles of the invention may also be employed tointegrate nanomaterials into electronic devices, in which thenanomaterials impart useful characteristics, e.g., as electrodes and orother active elements in photovoltaic devices and the like, or they maybe used to integrate these nanomaterials into physical structures, e.g.,composites, or biological structures, e.g., tissue. Synthesis ofnanofibers on a porous or curved substrate can facilitate production oflarge numbers and/or a high density of long, unbranched nanofibers foruse in any of a variety of applications.

Thus, a first general class of embodiments provides methods of producingnanofibers. In the methods, a substrate comprising a) a plurality ofapertures disposed therethrough, the substrate comprising an overallsurface area that includes an interior wall surface area of theplurality of apertures, or b) a curved surface is provided. A pluralityof nanofibers is synthesized on the substrate, wherein the resultingnanofibers are attached to at least a portion of the overall surfacearea of the substrate of a) or to at least a portion of the curvedsurface of b).

The substrate can comprise a solid substrate with a plurality of poresdisposed through it, a mesh (e.g., a metallic mesh, e.g., a meshcomprising a metal selected from the group consisting of: nickel,titanium, platinum, aluminum, gold, and iron), a woven fabric (e.g., anactivated carbon fabric), or a fibrous mat (e.g., comprising glass,quartz, silicon, metallic, or polymer fibers). As other examples, thesubstrate can comprise a plurality of microspheres (e.g., glass orquartz microspheres), a plurality of fibers, e.g., glass or quartzfibers (e.g., microfibers, fiberglass, glass or quartz fiber filters),or a foam. In certain embodiments, the plurality of apertures in thesubstrate of a) have an effective pore size of less than 10 μm, lessthan 1 μm, less than 0.5 μm, or even less than 0.2 μm. In otherembodiments, the plurality of apertures in the substrate of a) have aneffective pore size of at least 25 μm, at least 50 μm, at least 100 μm,or more.

The nanofibers can comprise essentially any type of nanofibers. Incertain embodiments, the nanofibers comprise nanowires, and the methodscan include synthesizing the plurality of nanowires by depositing a goldcolloid on at least a portion of the overall surface area of thesubstrate of a) or on at least a portion of the curved surface of b) andgrowing the nanowires from the gold colloid, e.g., with a VLS synthesistechnique. The plurality of nanofibers optionally comprises asemiconductor material selected from group IV, group and group III-Vsemiconductors (e.g., silicon).

The methods optionally include surrounding or at least partiallyencapsulating the substrate and the resulting attached nanofibers with amatrix material; dissolving a soluble substrate following synthesis ofthe nanofibers on the substrate; forming a coating on the resultingnanofibers, wherein the coating is contiguous between adjacentnanofibers; disposing a layer of porous material on the resultingnanofibers (and optionally disposing the substrate on a second layer ofporous material, sandwiching the nanofiber-bearing substrate); and/orfunctionalizing the nanofibers (e.g., by attaching a chemical moiety ornanocrystal to their surface).

In one class of embodiments, yield of the resulting nanofibers having alength greater than 10 μm (e.g., greater than 20 μm, 30 μm, 40 μm, 50μm, or 60 μm) is at least 10% greater than yield of nanofibers of thatlength synthesized on a planar non-porous substrate of the same surfacearea using substantially the same growth process. The yield from themethods is optionally at least 25%, 50%, 75%, or even 100% greater thanthe yield from growth on the planar non-porous substrate.

The nanofibers are optionally removed from the surface area of thesubstrate of a) or the curved surface of b) following synthesis of thenanofibers, e.g., by sonicating the substrate, to produce a populationof detached nanofibers. In one class of embodiments, at least 10% of thenanofibers in the population of detached nanofibers have a lengthgreater than 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, or 60 μm, while at most50% of the nanofibers have a length less than 10 μm.

Articles or populations of nanofibers produced by the methods formanother feature of the invention. Thus, one exemplary class ofembodiments provides an article comprising a substrate having a curvedsurface, and a plurality of nanofibers (e.g., nanowires) attached to atleast a portion of the curved surface of the substrate. The substratecan comprise, e.g., a plurality of microspheres or one or more glassfiber, quartz fiber, metallic fiber, polymer fiber, or other fiber.

As for the embodiments above, the plurality of nanofibers optionallycomprises a semiconductor material selected from group IV, group II-VIand group III-V semiconductors (e.g., silicon). Optionally, at least 10%of the nanofibers present on the curved surface have a length greaterthan 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, or 60 μm, while at most 50% ofthe nanofibers present on the curved surface have a length less than 10μm. The nanofibers can be preformed and deposited on the substrate toproduce the article, or the plurality of nanofibers can be attached tothe portion of the curved surface by having been grown on the portion ofthe curved surface. The article optionally includes a matrix materialsurrounding at least a portion of the curved surface and plurality ofnanofibers. Devices or compositions including the article form anotherfeature of the invention, for example, an implantable medical devicecomprising an article of the invention, e.g., attached to and coveringat least a portion of the surface of the implantable medical device.

Another general class of embodiments provides methods of stabilizingnanofibers (e.g., nanowires). In the methods, a population of nanofibersis provided, and a coating is formed on the nanofibers. The coating iscontiguous between adjacent nanofibers in the population. A firstmaterial comprising the nanofibers is optionally different from a secondmaterial comprising the coating. In one class of embodiments, thecoating comprises a carbide, a nitride, or an oxide, e.g., an oxide ofsilicon, titanium, aluminum, magnesium, iron, tungsten, tantalum,iridium, or ruthenium, or an oxide of the material comprising thenanofibers. In another class of embodiments, the nanofibers arecomprised of silicon and the coating is comprised of polysilicon.

The population of nanofibers is optionally provided by synthesizing thenanofibers on a surface of a substrate. The methods can includefunctionalizing the coating with a chemical binding moiety, ahydrophobic chemical moiety, a hydrophilic chemical moiety, or the like.

Populations of nanofibers formed by the methods are another feature ofthe invention. One general class of embodiments provides a population ofnanofibers that includes nanofibers (e.g., nanowires) and a coating onthe nanofibers, wherein the coating is contiguous between adjacentnanofibers in the population. As for the methods described above, afirst material comprising the nanofibers is optionally different from asecond material comprising the coating. In one class of embodiments, thecoating comprises a carbide, a nitride, or an oxide, e.g., an oxide ofsilicon, titanium, aluminum, magnesium, iron, tungsten, tantalum,iridium, or ruthenium, or an oxide of the material comprising thenanofibers. In another class of embodiments, the nanofibers arecomprised of silicon and the coating is comprised of polysilicon. Thecoating can be functionalized with a chemical binding moiety, ahydrophobic chemical moiety, a hydrophilic chemical moiety, or the like.The nanofibers are optionally attached to a substrate. In oneembodiment, the nanofibers are attached to and cover at least a portionof a surface of an implantable medical device.

Yet another general class of embodiments provides an article comprisinga substrate having a plurality of apertures disposed therethrough, thesubstrate comprising an overall surface area that includes an interiorwall surface area of the plurality of apertures, and a plurality ofnanofibers attached to at least a portion of the overall surface area ofthe substrate. The substrate can comprise, for example, a solidsubstrate (e.g., a silica based wafer, a metallic plate, or a ceramicsheet or plate) while the plurality of apertures comprises a pluralityof pores disposed through the solid substrate. As another example, thesubstrate can comprise a mesh, e.g., a polymer mesh or a metallic mesh(comprising, e.g., nickel, titanium, platinum, aluminum, gold, or iron).As yet another example, the substrate can comprise a woven fabric, e.g.,a fabric comprising fiberglass, carbon fiber, or a polymer (e.g.,polyimide, polyetherketone, or polyaramid). As yet another example, thesubstrate can comprise a fibrous mat, e.g., a fibrous mat comprisingsilica based fibers (e.g., glass and silicon), metallic fibers, orpolymer fibers.

In certain embodiments, the plurality of apertures have an effectivepore size of less than 10 μm, for example, less than 1 μm, less than 0.5μm, or less than 0.2 μm. In other embodiments, for example, embodimentsin which synthesis of long unbranched nanofibers are desired, theplurality of apertures have an effective pore size of at least 25 μm, atleast 50 μm, at least 100 μm, or more.

The nanofibers (e.g., nanowires) can comprise essentially any suitablematerial. For example, the plurality of nanofibers can comprise asemiconductor material selected from group IV, group II-VI and groupIII-V semiconductors, e.g., silicon. The nanofibers can be pre-formedand deposited on the substrate, or they can be attached to the portionof the overall surface area of the substrate by having been grown on theportion of the surface area. The plurality of nanofibers is optionallyelectrically coupled to the substrate. The plurality of nanofibers canbe functionalized with a chemical binding moiety, e.g., a hydrophobicchemical moiety.

In one class of embodiments, a matrix material surrounds or at leastpartially encapsulates the substrate and plurality of nanofibers. Thematrix material can at least partially intercalate into the apertures.In one embodiment, the matrix material and the plurality of nanofibershave a type-II energy hand-gap offset with respect to each other. Thematrix material optionally comprises a polymer, for example, apolyester, an epoxy, a urethane resin, an acrylate resin, polyethylene,polypropylene, nylon, or PFA. In one aspect, the invention providesimplantable medical devices. For example, an implantable medical devicecan include an article of the invention attached to and covering atleast a portion of a surface of the implantable medical device.

In one class of embodiments, the substrate comprises activated carbon,e.g., an activated carbon fabric. At least a first population ofnanocrystals can be attached to the nanofibers, for example,nanocrystals comprising a material selected from the group consistingof: Ag, ZnO, CuO, Cu₂O, Al₂O₃, TiO₂, MgO, FeO, and MnO₂. At least asecond population of nanocrystals is optionally also attached to thenanofibers, where the nanocrystals of the second population comprise adifferent material than do the nanocrystals of the first population. Incertain embodiments, the nanofibers are functionalized with a chemicalmoiety, e.g., a chemical moiety that absorbs or decomposes a non-organicgas. Preferred nanofibers in these embodiments include carbon nanotubesand silicon nanowires. An article of clothing can comprise thenanofiber-enhanced substrate of the invention.

Various techniques can be used to protect the nanofiber bearingsubstrate. For example, in one class of embodiments, the substrate(e.g., a woven fabric) comprises a first surface, and the articlefurther comprises a first layer of porous material disposed on the firstsurface of the substrate. Optionally, the substrate comprises a secondsurface, and the article also includes a second layer of porous materialdisposed on the second surface of the substrate, whereby the substrateis sandwiched between the first and second layers of porous material. Asanother example, the article can include a coating on the nanofibers,which coating is contiguous between adjacent nanofibers.

Yet another general class of embodiments provides methods of producing avapor absorbing fabric. In the methods, a porous fabric substrate thatcomprises a plurality of apertures disposed therethrough is provided.The substrate comprises an overall surface area that includes aninterior wall surface area of the plurality of apertures. A plurality ofnanofibers attached to at least a portion of the overall surface area ofthe fabric substrate is also provided, and the nanofibers arefunctionalized with a moiety that absorbs or decomposes at least oneorganic or non-organic gas, thereby producing a vapor absorbing fabric.

The fabric is preferably an activated carbon fabric. The nanofibers canbe functionalized with a chemical moiety that absorbs or decomposes atleast one non-organic gas. Preferably, the nanofibers are functionalizedby attaching at least a first population of nanocrystals to thenanofibers, which first population of nanocrystals comprises a firstmaterial that absorbs or decomposes at least one non-organic gas. Thevapor absorbing fabric can be incorporated into an article of clothingor other protective apparatus.

A related class of embodiments also provides methods of producing avapor absorbing fabric. In the methods, a porous fabric substrate thatcomprises a plurality of apertures disposed therethrough is provided(e.g., a mesoporous carbon fabric). The substrate comprises an overallsurface area that includes an interior wall surface area of theplurality of apertures. A plurality of nanocrystals is attached to atleast a portion of the overall surface area of the fabric substrate,which nanocrystals absorb or decompose at least one non-organic gas,thereby producing the vapor absorbing fabric.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 Panels A and B show a schematic illustration of a poroussubstrate having nanowires attached to its surfaces.

FIG. 2 Panels A and B show a schematic illustration of nanowiresattached to the interior wall portions of a porous substrate material.

FIG. 3 Panels A and B show a schematic illustration of the articles ofthe invention incorporated in a filtration cartridge.

FIG. 4 Panels A and B show a schematic illustration of a layered textilethat incorporates a substrate of the invention as a semi-permeablemoisture barrier for use in, e.g., outdoor clothing.

FIG. 5 Panels A and B show a schematic illustration of the articles ofthe invention incorporated in a self adhesive, moisture repellantbandage.

FIG. 6 Panels A and B are schematic illustrations of the substratematerial of the invention incorporated into a photovoltaic device.

FIG. 7 is a schematic illustration of an article of the invention usedas a lattice for incorporation into a composite matrix for use as, e.g.,a dielectric layer.

FIG. 8 Panels A and B schematically illustrate separation mediaincorporating the substrates of the invention in conjunction with columnapparatus for performing chromatographic separations.

FIG. 9 Panels A and B show electron micrographs of cross-fused or linkednanowires creating an independent mesh network as used in certainaspects of the present invention.

FIG. 10 schematically illustrates a process for producing a cross-linkednanowire mesh network for use either in conjunction with or independentfrom an underlying porous, e.g., macroporous, substrate.

FIG. 11 illustrates a composite material that employs the poroussubstrates of the invention disposed within a matrix material.

FIG. 12 Panels A and B illustrate an example of the nanofiber bearing,porous substrates of the invention.

FIG. 13 schematically illustrates a nanofiber-enhanced fabric for use,e.g., in protective clothing or apparatus.

FIG. 14 schematically illustrates protection of a nanofiber-bearingsubstrate by disposing it between layers of porous material.

FIG. 15 Panel A depicts an electron micrograph of reticulated aluminum.Panel B depicts an electron micrograph of nanowires grown on areticulated aluminum substrate.

FIG. 16 depicts silicon nanowires grown on quartz fiber filters (PanelsA-B), grown on quartz fiber filters and removed from the substrate bysonication (Panels C-D), and grown on a glass fiber (Panel E).

FIG. 17 Panel A depicts simulated nanowires growing on a 5 μm diameterfiber. Panel B depicts a graph of the collisions per nanowire as afunction of fiber radius for simulated nanowire growth on a fiber.

FIG. 18 Panel A depicts an electron micrograph of carbon black powderedparticles having gold colloid particles deposited thereon.

FIG. 18 Panels B and C depict an electron micrograph of siliconnanowires grown from the carbon black supported gold colloid particlesof FIG. 18 Panel A.

DETAILED DESCRIPTION I. General Description of the Invention

The present invention generally provides, inter alia, novel articles andcompositions that employ nanowire surfaces or surface portions to impartunique physical, chemical and electrical properties. In particular, thepresent invention is directed, in part, to porous substrates that havenanowires attached to at least a portion of the overall surfaces of theporous substrates in order to provide materials that have a wide rangeof unique and valuable properties for a wide range of differentapplications.

The application of nanowires to the various surfaces of poroussubstrates not only improves the performance of porous substrates inapplications where they are already used, but also improves performanceof substrate materials in a number of other different applications,where such porous substrates may or may not conventionally be employed.

By way of example, incorporation of nanowire enhanced surfaces inmembranes or other semi-permeable barriers can enhance filtrationefficiencies. In particular, by providing nanowires within the pores ofexisting membranes or other permeated layers, one can provide higherfiltration efficiencies without the expected increase in pressure dropacross the filter (see Grafe et al., Nanowovens in Filtration-FifthInternational Conference, Stuttgart, Germany, March 2003). Relatedly,such nanofibers may be used to impart alternate properties to suchbarriers, e.g., breathable moisture repellant barriers,antibacterial/antiseptic barriers. Such barriers would be widelyapplicable in the outdoor clothing industry but would also beparticularly useful as bandages or surgical dressings due to theirpermeability to oxygen but impermeability to moisture or particlesincluding bacteria, as well as the use of antimicrobial nanofibers. Thislatter application is particularly interesting in light of the dryadhesive characteristics of nanowire/nanofiber enhanced surfaces (see,e.g., U.S. Pat. No. 7,056,409, incorporated herein by reference in itsentirety for all purposes). Relatedly, such nanofiber enhanced surfacescan also be used in the construction of chemical and/or biologicalprotective barriers, e.g., clothing, optionally permeable to moisturebut absorbing chemical vapors.

While some researchers have proposed depositing nanofibers ontomembranes to achieve higher surface areas, the ability to attach fibersto the surface, and particularly to grow such fibers in situ, providesnumerous advantages over simple deposition of fibers. In particular, inmerely depositing fibers on membranes, it is difficult to get uniform orcomplete, e.g., penetrating, coverage of the fibers over the totalsurface area of the membrane, whereas in situ growth methods give farbetter coverage of interior surfaces, and thus provide much greatersurface area for the membrane or barrier. Additionally, such methodsprovide for varied orientations of such fibers from the surfaces towhich they are attached, i.e., having fibers extend from the surface asopposed to laying flat against the surface.

In addition to improving the function of porous substrates, the use ofporous substrates in conjunction with nanofibers/nanowires also providesa unique, ultra high surface area material that can be used in a widevariety of applications that may have little to do with the use ofporous substrates, per se. For example, ultra high surface areaelectrical components may have a variety of applications as electrodesfor interfacing with, e.g., biological tissue (e.g., in pacemakers),coverings for other biological implants as tissue lattice oranti-infective barriers for catheters, or the like.

In still other applications, porous substrates provide a uniquesynthesis lattice for providing dense populations ofnanofibers/nanowires for use in a variety of different applications,e.g., for use in composite films, etc. Such films may generally beapplied as semiconductive composites, dielectric films, active layersfor electronic or photoelectric devices, etc.

In still other applications, porous substrates provide a uniquesynthesis lattice for synthesizing nanofibers, particularly long,unbranched nanowires at high yield and/or density.

A broad range of potential applications exists for these techniques,materials, and articles and will be apparent to one of ordinary skill inthe art upon reading the instant disclosure.

II. Articles of the Invention, Structure and Architecture

As noted above, in one aspect, the articles of the invention incorporateporous substrates as a foundation of the article. The porous substratesused in accordance with the present invention typically include any of avariety of solid or semisolid materials upon which the nanowires may beattached, but through which apertures exist. As such, these substratesmay include solid contiguous substrates, e.g., plates, films, or wafers,that may be flexible or rigid, that have apertures disposed throughthem, e.g., stamped or etched metal or inorganic perforated plates,wafers, etc., porated or perforated films, or the substrate may includeaggregates of solid or semisolid components e.g., fibrous mats, meshscreens, amorphous matrices, composite materials, woven fabrics, e.g.,fiberglass, carbon fiber, polyaramid or polyester fabrics, or the like.As will be apparent, any of a wide variety of different types ofmaterials may comprise the substrates, including organic materials,e.g., polymers, carbon sheets, etc., ceramics, inorganic materials,e.g., semiconductors, insulators, glasses, including silica basedmaterials (e.g., silicon, SiO₂), etc., metals, semimetals, as well ascomposites of any or all of these.

Additionally, substrates, e.g., rigid or solid substrates, may beengineered to have additional topographies, e.g., three dimensionalshapes, such as wells, pyramids, posts, etc. on their surface to furtherenhance their effectiveness, e.g., provide higher surface areas, channelfluids or gases over them, provide prefiltration in advance of thefiltration provided by the porous substrate, per se, etc. Additionally,although referred to as including a porous substrate, it will beappreciated that in application, multiple substrates may be providedtogether in a single article, device or system. Further, althoughdescribed and exemplified primarily as planar porous substrates, it willbe appreciated that the porous substrates may be fabricated into any ofa variety of shapes depending upon the application, including non-planarthree dimensional shapes, spheres, cylinders, disks, cubes, blocks,domes, polyhedrons, etc. that may be more easily integrated into theirdesired application. Substrates, e.g., planar sheet substrates, areoptionally rigid or flexible.

Examples of metal substrates include steel/iron, nickel, aluminum,titanium, silver, gold, platinum, palladium, or virtually any metalsubstrate that imparts a desirable property to the finished article,e.g., conductivity, flexibility, malleability, cost, processibility,etc. In certain preferred aspects, a metal wire mesh or screen is usedas the substrate. Such meshes provide relatively consistent surfaces ina ready available commercial format with well defined screen/pore andwire sizes. A wide variety of metal meshes are readily commerciallyavailable in a variety of such screen/pore and wire sizes.Alternatively, metal substrates may be provided as perforated plates,e.g., solid metal sheets through which apertures have been fabricated.Fabricating apertures in metal plates may be accomplished by any of anumber of means. For example, relatively small apertures, e.g., lessthan 100 μm in diameter, as are used in certain aspects of theinvention, may be fabricated using lithographic and preferablyphotolithographic techniques. Similarly, such apertures may befabricated using laser based techniques, e.g., ablation, laser drilling,etc. For larger apertures, e.g., greater than 50-100 μm, moreconventional metal fabrication techniques may be employed, e.g.,stamping, drilling or the like.

Polymeric and inorganic substrates may be similarly structured to themetal substrates described above, including mesh or screen structures,fibrous mats or aggregates, e.g., wools, or solid substrates havingapertures disposed through them. In terms of polymeric substrates,again, the primary selection criteria is that the substrate operate inthe desired application, e.g., is resistant to chemical, thermal orradiation or other conditions to which it will be exposed. In preferredaspects the polymeric substrate will also impart other additional usefulcharacteristics to the overall article, such as flexibility,manufacturability or processibility, chemical compatibility orinertness, transparency, light weight, low cost, hydrophobicity orhydrophilicity, or any of a variety of other useful characteristics.Particularly preferred polymeric substrates will be able to withstandcertain elevated environmental conditions that may be used in theirmanufacturing and/or application, e.g., high temperatures, e.g., inexcess of 300 or 400° C., high salt, acid or alkaline conditions, etc.In particular, polymers that tolerate elevated temperatures may beparticularly preferred where the nanowires are actually grown in situ onthe surface of the substrate, as such synthetic processes often employhigher temperature synthetic processes, e.g., as high as 450° C.Polyimide polymers, polyetherketone, polyaramid polymers and the likeare particularly preferred for such applications. Those of skill in theart will recognize a wide range of other polymers that are particularlysuitable for such applications. Alternatively, lower temperature fibersynthesis methods may also be employed with a broader range of otherpolymers. Such methods include that described by Greene et al.(“Low-temperature wafer scale production of ZnO nanowire arrays”, L.Greene, M. Law, J. Goldberger, F. Kim, J. Johnson, Y. Zhang, R.Saykally, P. Yang, Angew. Chem. Int. Ed. 42, 3031-3034, 2003), orthrough the use of PECVD, which employs synthesis temperatures ofapproximately 200° C. In the case where the porous substrate is merelythe recipient of nanofibers already synthesized, e.g., where thesubstrate is either to be coupled to the nanowires or is to act as amacroporous support for the nanowires, a much wider variety of poroussubstrates may be employed, including organic materials, e.g., organicpolymers, metals, ceramics, porous inorganics, e.g., sintered glass,which would include a variety of conventionally available membranematerials, including cellulosic membranes, e.g., nitrocellulose,polyvinyl difluoride membranes (PVDF), polysulfone membranes, and thelike.

In some cases, the porous substrate may comprise a soluble material,e.g., cellulose, or the like. Following attachment of the nanofibers,and optionally placement of the overall substrate into its ultimatedevice configuration, the supporting porous substrate may be dissolvedaway, leaving behind an interwoven mat or collection of nanofibers. Forexample, a soluble mesh may be provided with nanofibers attached to itsoverall surfaces or interior wall surfaces as described herein. The meshmay then be rolled into a cylindrical form and inserted into acylindrical housing, e.g., a column for separations applications. Thesupporting mesh is then dissolved away to yield the column packed withnanofibers. Further, as described above, the porous matrix may compriseany of a number of shapes, and be soluble as well, so as to yield any ofa variety of shapes of aggregations of fibers, once the substrate isdissolved.

As noted above, the apertures of the substrates used herein typicallyare defined in terms of their effective pore size or “effectiveporosity”. Although described as apertures or pores, it will beappreciated that the term “aperture” or “pore” when used in the contextthat it is disposed through a substrate, refers simply to a contiguouspathway or passage through a substrate material, whether that materialbe a single solid piece of substrate material or a mesh or mat ofaggregated pieces of substrate material. Thus, such “apertures” or poresdo not need to represent a single passage, but may constitute multiplepassages strung together to form the contiguous path. Likewise, anaperture or pore may simply represent the space between adjacentportions of substrate material, e.g., fibers, etc. such that the spacesprovide a contiguous path through the material. For purposes of theinvention, pore or aperture size, in the absence of any nanofibersdisposed thereon, will typically vary depending upon the nature of theapplication to which the material is to be put.

For example, filtration applications will typically vary pore sizedepending upon the nature of the particles or other material to befiltered, ranging from tens to hundreds of microns or larger for coarserfiltration operations to submicron scale for much finer filtrationapplications, e.g., bacterial sterilizing filters. Similarly forsemi-permeable barrier applications, such pores will typically varydepending upon the type of permissible permeability is sought. Forexample, breathable moisture barriers may have pore sizes from tens ofmicrons to the submicron range, e.g., 0.2 μm, or smaller. In some cases,it may be desirable to have an effective pore size that is less than 100nm, and even less than 20 nm, so as to block passage of biologicalagents, e.g., bacteria and viruses.

The articles and substrates described herein may include nanowiressubstantially on any and all surfaces of the substrate materialincluding both exterior surfaces and the surfaces that are within thepores. Together, these surfaces upon which nanowires may be disposed arereferred to herein as the “overall surface” of the substrate material,while the wall surfaces that are disposed upon the interior walls of thepores are generally referred to herein as the “interior wall surfaces”of the substrate material or pores. As will be clear to one of ordinaryskill in reading the instant disclosure, a reference to a surface as aninterior wall surface for certain embodiments, e.g., in the case of afibrous mat or wool like substrate does not necessarily denote apermanent status of that surface as being in the interior portion of apore or aperture as the basic flexibility and/or malleability of certainsubstrate materials may provide the ability to shift or move the variousportions of the substrate material's overall surface around.

As noted above, the substrates of the invention gain significant uniqueproperties by incorporating nanofibers or nanowires on their surfaces.For most applications, the terms “nanowire” and “nanofiber” are usedinterchangeably. However, for conductive applications, e.g., where thenanofibers' conductive or semiconductive properties are of interest, theterm “nanowire” is generally favored. In either instance, the nanowireor nanofiber generally denotes an elongated structure having an aspectratio (length:width) of greater than 10, preferably greater than 100 andin many cases 1000 or higher. These nanofibers typically have a crosssectional dimension, e.g., a diameter that is less than 500 nm andpreferably less than 100 nm and in many cases, less than 50 nm or 20 nm.

The composition of the nanofibers employed in the invention typicallyvaries widely depending upon the application to which the resultingsubstrate material is to be put. By way of example, nanofibers may becomprised of organic polymers, ceramics, inorganic semiconductors andoxides, carbon nanotubes, biologically derived compounds, e.g.,fibrillar proteins, etc. or the like. For example, in certainembodiments, inorganic nanofibers are employed, such as semiconductornanofibers. Semiconductor nanofibers can be comprised of a number ofGroup IV, Group III-V or Group II-VI semiconductors or their oxides.Particularly preferred nanofibers include semiconductor nanowires orsemiconductor oxide nanofibers.

Typically, the nanofibers or nanowires employed are produced by growingor synthesizing these elongated structures on substrate surfaces. By wayof example, Published U.S. Patent Application No. US-2003-0089899-A1discloses methods of growing uniform populations of semiconductornanowires from gold colloids adhered to a solid substrate using vaporphase epitaxy. Greene et al. (“Low-temperature wafer scale production ofZnO nanowire arrays”, L. Greene, M. Law, J. Goldberger, F. Kim, J.Johnson, Y. Zhang, R. Saykally, P. Yang, Angew. Chem. Int. Ed. 42,3031-3034, 2003) discloses an alternate method of synthesizing nanowiresusing a solution based, lower temperature wire growth process. A varietyof other methods are used to synthesize other elongated nanomaterials,including the surfactant based synthetic methods disclosed in U.S. Pat.Nos. 5,505,928, 6,225,198 and 6,306,736, for producing shorternanomaterials, and the known methods for producing carbon nanotubes,see, e.g., US-2002/0179434 to Dai et al. As noted herein, any or all ofthese different materials may be employed in producing the nanofibersfor use in the invention. For some applications, a wide variety of groupIII-V, II-VI and group IV semiconductors may be utilized, depending uponthe ultimate application of the substrate or article produced. Ingeneral, such semiconductor nanowires have been described in, e.g.,US-2003-0089899-A1, incorporated herein above. In certain preferredembodiments, the nanowires are selected from a group consisting of: Si,Ge, Sn, Se, Te, B, diamond, P, B—C, B—P(BP6), B—Si, Si—C, Si—Ge, Si—Sn,Ge—Sn, BN, BP, BAs, AlN, MP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP,InAs, InSb, ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, BeS,BeSe, BeTe, MgS, MgSe, GeS, GeSe, GeTe, SnS, SnSe, SnTe, PhO, PbS, PbSe,PbTe, CuF, CuCl, CuBr, CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂,CdSnAs₂, ZnSnSb₂, CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se,Tc)₂, Si₃N₄, Ge₃N₄, Al₂O₃, (Al, Ga, In)₂(S, Se, Te)₃, Al₂CO, and anappropriate combination of two or more such semiconductors. Thenanofibers optionally comprise a gold tip.

In the cases of semiconductor nanofibers, and particularly those for usein electrical or electronic applications, the nanofibers may optionallycomprise a dopant from a group consisting of: a p-type dopant from GroupIII of the periodic table; an n-type dopant from Group V of the periodictable; a p-type dopant selected from a group consisting of: B, Al andIn; an n-type dopant selected from a group consisting of: P, As and Sb;a p-type dopant from Group II of the periodic table; a p-type dopantselected from a group consisting of: Mg, Zn, Cd and Hg; a p-type dopantfrom Group IV of the periodic table; a p-type dopant selected from agroup consisting of: C and Si; or an n-type dopant is selected from agroup consisting of: Si, Ge, Sn, S, Se and Te.

In some cases, it may be desirable to utilize nanofibers that have aself sterilizing capability, e.g., in semipermeable bandage, clothing,filtration or other applications. In such cases, the nanofibers may befabricated from, e.g., TiO₂, which upon exposure to UV light oxidizesorganic materials to provide a self cleaning functionality (See, e.g.,US Patent Application Publication No. 20060159916, and incorporatedherein by reference in its entirety for all purposes).

Additionally, such nanofibers may be homogeneous in their composition,including single crystal structures, or they may be comprised ofheterostructures of different materials, e.g., longitudinalheterostructures that change composition over their length, or coaxialheterostructures that change composition over their cross section ordiameter. Such coaxial and longitudinal heterostructured nanowires aredescribed in detail in, e.g., Published International Patent ApplicationNo. WO 02/080280, which is incorporated herein by reference for allpurposes.

The nanowire portion of the articles of the invention are preferablysynthesized in situ, e.g., on the desired surface of the poroussubstrate. For example, in preferred aspects, inorganic semiconductor orsemiconductor oxide nanofibers are grown directly on the surface of theporous substrate using a colloidal catalyst based VLS(vapor-liquid-solid) synthesis method such as those described above. Inaccordance with this synthesis technique, the colloidal catalyst isdeposited upon the desired surface of the porous substrate (which insome cases may include the overall surface of the porous substrate). Theporous substrate including the colloidal catalyst is then subjected tothe synthesis process which generates nanofibers attached to the surfaceof the porous substrate. Other synthetic methods include the use of thincatalyst films, e.g., 50 nm, deposited over the surface of the poroussubstrate. The heat of the VLS process then melts the film to form smalldroplets of catalyst that form the nanofibers. Typically, this lattermethod may be employed where fiber diameter homogeneity is less criticalto the ultimate application. Typically, catalysts comprise metals, e.g.,gold, and may be electroplated or evaporated onto the surface of thesubstrate or deposited in any of a number of other well known metaldeposition techniques, e.g., sputtering etc. In the case of colloiddeposition, the colloids are typically deposited by first treating thesurface of the substrate so that the colloids adhere to the surface.Such treatments include those that have been described in detailpreviously, e.g., polylysine treatment, etc. The substrate with thetreated surface is then immersed in a suspension of colloid.

Alternatively, the nanofibers may be synthesized in another location anddeposited upon the desired surface of the porous substrate usingpreviously described deposition methods. For example, nanofibers may beprepared using any of the known methods, e.g., those described above,and harvested from their synthesis location. The free standingnanofibers are then deposited upon the relevant surface of the poroussubstrate. Such deposition may simply involve immersing the poroussubstrate into a suspension of such nanofibers, or may additionallyinvolve pretreating all or portions of the porous substrate tofunctionalize the surface or surface portions for fiber attachment. Avariety of other deposition methods are known, e.g., as described inU.S. Pat. No. 7,067,328, and 6,962,823, the full disclosures of whichare incorporated herein by reference in their entirety for all purposes.

Where nanofibers are desired to be attached primarily to the interiorwall portions of the surface of the porous substrate, such depositionmay be accomplished by growing the nanofibers in such locations or byselectively depositing the nanofibers in such locations. In the case ofin situ grown nanofibers, this may be accomplished by depositing a layerof another material on all of the exterior surfaces of the substrate,e.g., a resist, before depositing the colloids. Following immersion incolloid, the resist layer may be developed and removed to yieldsubstrate having colloid substantially only deposited on the interiorwall surfaces of the substrate.

FIGS. 1 and 2 schematically illustrate substrates according to thepresent invention. In particular, FIG. 1 shows a schematic illustrationof a porous nanowire carrying substrate of the invention. As shown inFIGS. 1A and 1B, a porous substrate 102 is provided. For purposes ofexemplification, a mesh or screen is employed as the porous substrate,although fibrous mats are also useful in such applications. As shown inFIG. 1B, nanofibers 104 are provided that are, at least in part,disposed on the internal wall portions 106 of the apertures or pores,and which extend into the void area 108 of the pores, yielding openingsor passages through the overall material that are somewhat morerestrictive or narrow than those provided by the underlying substrate,itself. As shown in FIG. 1, the nanofibers 104 are also disposed onother surface portions of the mesh (the overall surface).

FIGS. 2A and 2B schematically illustrate the case where nanofibers areprimarily disposed only on the interior wall portions of the aperturesthat define the pores. As shown, a perforated substrate 202 forms theunderlying porous substrate. A plurality of apertures 208 are fabricatedthrough the substrate 202, e.g., by punching etching or other knownfabrication methods. As shown in FIG. 2B, an expanded view of theaperture 208 is provided that details the presence of nanofibers 204attached to the interior wall portions 206 of the aperture. As shown,the nanofibers generally protrude away from the interior wall surface206. This is typically accomplished by growing the nanofibers, in situ,using a catalytic growth CVD process, whereupon the fibers grow awayfrom the surface upon which the catalyst is initially deposited. Othermethods may also be employed to deposit nanofibers on these interiorwall portions that may or may not result in the fibers protruding intothe void space of the apertures, including immersing the poroussubstrate in a suspension of nanofibers that are chemically able toattach to the surfaces of interest.

FIG. 12 shows a photograph of a silicon substrate that has pores orapertures disposed through it. Silicon nanofibers were grown over thesurface of the substrate, including within the pores. The substrate wasa 0.1 mm thick silicon wafer with regularly spaced 100 μm holes disposedthrough it. FIG. 12A shows a view of a larger area of the substrate,while FIG. 12 B shows a closer up view of the pore and substratesurface, as well as the nanofibers on those surfaces.

In alternative arrangements, the porous substrates may be employed insteps that are discrete from the synthesis process, and that employ theporous substrate as a capture surface for the nanofibers. In particular,nanofibers may be produced as suspensions or other collections orpopulations of free-standing, e.g., a population of discrete andindividual members, nanofibers. Such free standing nanofibers aregenerally produced from any of the aforementioned processes, butincluding a harvest step following synthesis whereby the nanofibers areremoved from a growth substrate and deposited into a suspending fluid orother medium or deposited upon a receiving substrate, or otherwise movedfrom a growth or synthesis environment into a manipulable environment,e.g., a fluid suspension. The population of nanowires is then depositedover a porous substrate to yield a mat of deposited nanofibers that forma micro or nanoporous network over the underlying porous substrate. Inaccordance with this aspect of the invention, the pores in the poroussubstrate are typically selected so that they are smaller than thelargest dimensions of the nanofibers to be deposited thereon, e.g., thelength of the nanofiber. For example, where nanofibers in a particularpopulation have an average length of approximately 10 μm, the pores inthe substrate will typically be smaller in cross section than 10 μm,e.g., less than 5 μm, less than 2 μm, or smaller. To ensure sufficientcapture of nanofibers, the largest cross section of the pore in theporous substrate will typically be less than 50% of the average largestdimension of the nanofiber population, generally the length, in somecases, less than 20% of such dimension, and in many cases, less than 10%of such dimension.

The nanofiber mat is then optionally fused or cross-linked at the pointswhere the various fibers contact each other, to create a more stable,robust and potentially rigid fibrous membrane. The void spaces betweenthe interconnected nanofibers form the porous network of the nanofibrousmat. The effective pore size of the mat will generally depend upon thedensity of the deposited nanofiber population that is deposited, as wellas the thickness of that layer, and to some extent, the width of thenanofibers used. All of these parameters are readily varied to yield amat having a desired effective porosity.

FIGS. 9A and 9B show electron micrographs of cross-linked nanofibrousmats that illustrate certain aspects of the invention. FIG. 9A shows apopulation of semiconductor nanofibers that were cross-linked throughvapor deposition of inorganic material, e.g., silicon. In particular, apopulation of silicon nanowires was prepared by a conventional synthesisscheme, e.g., silicon nanowires were grown at 480° C., from a goldcolloid catalyst, under SiH4 partial pressure, 1 torr, total pressure,30 torr for 40 minutes. After the growth was terminated by pumping outthe process gasses, the temperature of the substrate was ramped up to520° C. under 30 torr He. The process gases (SiH₄) were switched onagain once temperature was reached, and the resulting silicon depositioncross-linked the adjacent or contacting nanowires. The deposition timewas 10 minutes. As will be appreciated, separately harvested anddeposited nanofibers may be similarly crosslinked using this technique.

The nanofibers in FIG. 9B, on the other hand, were linked using apolymer deposition process that at least partially coated or encased thenanofibers to link them together. In particular, a PVDF polymer wassuspended along with the nanowires in acetone and sonicated. The acetonewas then evaporated to yield the encapsulated or crosslinked nanowiresor nanofibrous mat. As can be seen in each case, the network of siliconnanofibers, or nanowires, shows cross-linking at the intersections ofvarious nanofibers. Also as shown, the pores created by the interwovennanofibers are defined by the void space between the nanofibers.

As noted above, the alternative aspects of the invention may beaccomplished by simply depositing nanowires upon a receiving orsupporting substrate such that the nanofibers are overlaying each otherto form a mat, and preferably a dense nanofiber mat. In general, thisprocess is simplified by using a porous supporting substrate such thatthe nanofibers may be captured upon the upper surface of the poroussupporting substrate while the medium in which the nanofibers wereoriginally disposed is allowed to pass through the pores, essentiallyfiltering the nanofibers with the substrate and densely depositing thenanofibers on the surface of the substrate. The resulting fibrous mat isthen treated to crosslink the fibers at the points where they contact orare sufficiently proximal to each other.

The process for such mat formation is schematically illustrated in FIG.10. In particular, a nanofiber population 1000 is provided as asuspension 1002, where the nanofibers may be suspended in liquid, gas,or simply provided as a free flowing population or powder. The nanofiberpopulation is then deposited or poured onto a porous substrate 1004. Thenanofiber population 1000 is then retained upon the upper surface 1006of the porous substrate 1004, at which point it forms an overlaying mat1008 of nanofibers supported by substrate 1004. The mat 1008 is added toby depositing additional nanofibers onto the substrate. As notedpreviously, any medium in which the nanofibers are suspended freelypasses through pores 1010 in the porous substrate 1004, allowing thenanofibers to pack densely against the upper surface 1006 of the poroussubstrate 1004.

Once the nanofiber mat 1008 is of the desired thickness and fiberdensity, the mat may be readily employed upon its supporting macroporoussubstrate, e.g., as a filter membrane or other semipermeable layer.However, in preferred aspects, the nanofibrous mat is treated (asindicated by arrows 1014) to crosslink the nanofibers at theirrespective contact points to form couplings 1012 between the nanofibersin the mat, as shown in the expanded view. The use of crosslinkednanofibers has been described for ultra high surface area applications(See, e.g., commonly owned U.S. Patent Application Publication No.20060159916, and incorporated herein by reference in its entirety forall purposes). Crosslinking, as noted previously, may be accomplished bya number of means, including thermal fusing, chemical surfacemodification/crosslinking, encapsulation or coating. Thermal fusingmethods may vary depending upon the makeup of the nanofibers, withpolymeric nanofibers being, fused at substantially lower temperaturesthan metal or inorganic semiconductor nanofibers.

Nanofibers may also include surface chemical groups that may formchemical crosslinks in order to cross-link the underlying nanofibers.For example, polymeric materials, such as polyacrylamide or polyethyleneglycol groups, may be readily coupled to the surfaces of nanofibers,e.g., through well known silane and/or pegylation chemistries. Wellknown polymer crosslinking techniques are then used to crosslink thenanofibers. Similarly, epoxide, acrylate or other readily availablereactive groups may be provided upon the surface of the nanofibers thatallow thermal curing, optical curing, e.g., UV, or other chemicalinteraction and coupling between adjacent, contacting nanofibers toprovide the crosslinking.

In another aspect, the nanofibrous mat may be crosslinked together usinga polymer coating or encapsulation technique that locks the variousnanofibers into position. For example, vapor deposition techniques maybe employed to vapor deposit thin polymer layers over the nanofiberportions of the mat, effectively cementing the nanofibers into position.Examples of such polymers include, e.g., PTFE, PVDF, parylene, and thelike. A wide variety of other polymeric materials may optionally beemployed using a liquid deposition or an in situ polymerization and/orcrosslinking techniques, e.g., as described above. As will beappreciated, polymeric crosslinking may provide certain benefits overthermal and/or chemical crosslinking in terms of pliability of theresulting mat of material.

Once the nanofibrous mat is crosslinked, it may be employed along withthe underlying macroporous substrate, e.g., as a backing, or it may beseparated from the substrate to yield an independent nanofibrousmembrane, e.g., membrane 1016. As will be readily appreciated, largerarea nanofiber layers may be produced using conventionally availableprocesses, including drum or belt filter techniques where a large area,continuous macroporous substrate layer, e.g., in a belt or as a surfaceof a drum, is used to retain nanofiber layers, which layers arecrosslinked or otherwise treated as described herein. Such processes maybe configured in a continuous or large area batch mode operation inorder to provide extremely large amounts of the fibrous layer material,e.g., for use in clothing, outdoor fabrics, e.g., tents, and other highvolume applications.

In one embodiment, an article of the invention includes a nanofibrousmat that comprises a plurality of overlaid nanofibers, wherein saidplurality of nanofibers are crosslinked together at points where suchnanofibers contact or are proximal to others of said nanofibers, to forma semipermeable layer. The nanofibrous mat is optionally deposited upona surface of a porous substrate, with the porous substrate andnanofibrous mat forming a semipermeable layer. At least a portion of thenanofibers optionally comprise an attached hydrophobic moiety.

In one embodiment, methods of producing a contiguous population ofnanofibers are provided. In the methods, a porous substrate having anoverall surface area is provided, as is a plurality of nanofibersattached to the overall surface area of the porous substrate. A relatedembodiment also provides methods of producing a contiguous population ofnanofibers. The methods include providing a porous substrate having anupper surface and a plurality of pores disposed through the poroussubstrate, wherein each of said pores has an effective pore size;depositing a plurality of nanofibers onto the upper surface of theporous substrate, said nanofibers having at least one dimension greaterthan the effective pore size, such that the nanofibers are retained uponthe upper surface as a nanofibrous mat; and crosslinking individualnanofibers in the plurality of nanofibers with other individualnanofibers of the plurality of nanofibers to produce a contiguousnanofiber population.

III. Applications

As alluded to herein, the porous substrates of the invention havingnanofibers attached to portions of their surfaces have myriadapplications that take advantage of a wide variety of particularlyinteresting properties of such materials. In certain applications, thepresence of nanowires provides porous materials with enhancedproperties. In other applications, the combination of porous substratesand nanowires provides materials having substantially new properties andusefulness.

A. Semi-Permeable Barriers.

In a first particularly preferred application, the porous substrates ofthe invention are useful as semi-permeable barriers. Semi-permeablebarriers, in general, also find a wide variety of different applicationsdepending upon their level of permeability, cost, etc. For example, suchbarriers may be permeable to gas and not liquid, or to air or gas andnot particulate matter. Still further, such semi-permeable barriers mayprovide antiseptic or antibacterial properties to their applications.

In one aspect, the invention provides a semipermeable membrane includinga porous substrate having a plurality of apertures disposedtherethrough, the porous substrate having an overall surface thatincludes an interior wall surface of the apertures, and a plurality ofnanofibers deposited upon or attached to at least a portion of theoverall surface of the porous substrate, wherein the nanofibers andapertures together define a pore through the semipermeable membrane, thepore being permeable to one or more materials and not permeable to oneor more different materials. The nanofibers can be attached to ordeposited on at least a portion of the overall surface of the poroussubstrate. Individual nanofibers can be crosslinked with otherindividual nanofibers to provide a crosslinked nanofibrous mat on thesurface of the porous substrate. The nanofibers optionally comprise ahydrophobic moiety coupled thereto, rendering the pore permeable to gasbut not permeable to liquid water. The pore can have an effective poresize that excludes particles of a first particle size while permittingpassage of particles of a second particle size, smaller than the firstparticle size. The effective pore size can be, e.g., smaller than 10 μm,smaller than 1 μm, less than 0.2 μm, less than 100 nm, or even less than20 nm. The membrane can be incorporated into a variety of articles. Forexample, a filter cartridge can include a semipermeable membrane of theinvention disposed within a housing having an inlet passage in fluidcommunication with a first side of the semipermeable membrane and anoutlet side in fluid communication with a second side of thesemipermeable membrane. In one embodiment, the housing is coupled to abreathing mask to filter breathing air. As another example, an articleof clothing can include a semipermeable membrane of the invention, e.g.,layered with at least a second fabric layer.

1. Filtration

In their simplest aspect, semi-permeable barriers are used as filtrationmedia for separating gases or liquids from particulate matter. Forexample, there are a wide range of different filtration optionsavailable for e.g., air filtration, from simple consumer filtrationneeds, e.g., home furnaces, air conditioners, air purifiers, to moredemanding filtration needs, e.g., HEPA filtration for industrial use,hazardous materials filtration for protective gear, clean roomapplications, automotive applications, etc. For liquid applications,such filters may provide water purification, particulate separation forfuels and lubricants for industrial or consumer machinery, e.g.,automobiles, etc.

In accordance with the filtration applications of the present invention,porous substrates are used as the foundation for the filtration media tobe produced. Nanofibers are then provided on the surfaces overall orsubstantially only interior surfaces to further enhance the filtrationcapabilities of the underlying porous foundation. In particular, one ofthe key areas that are sought for improvement in filtration media is theability to increase the filtration efficiency, e.g., reduce the poresize or increase the overall capacity/lifetime of a filter, withoutyielding a substantial increase in the pressure drop, which could leadto early filter failure/clogging, higher energy demands, etc.

The enhancements brought by the present invention include effectivelydecreasing the pore size of the filter without substantially increasingthe pressure drop across a filter. In particular, the present inventionprovides porous substrates having nanofibers disposed within the poresof the substrate to provide additional filtration by modifying theeffective pore size. Nanofibers, because of their extremely small size,are particularly useful in these applications due to their ability tosubstantially increase the surface area within a pore withoutsubstantially increasing the volume of material disposed within thatpore, this increasing the filtration efficiency without decreasing theflow through the filter media. Fluids or gases are then passed throughthe porous substrate to separate particulate materials from the carrierliquid or gas.

One example of a filtration cartridge, e.g., for a filter mask, gas lineor the like, is illustrated in FIGS. 3A and 3B, As shown in FIG. 3A, afilter cartridge 300 includes a main housing 302 having a filter layer304 disposed within the housing. A filter support 306 is typically alsoincluded on the low pressure side of the filter layer to providestructural support to the filter layer. The filter layer typicallyincludes an inlet or high pressure side 308 and a low pressure or outletside 310. Gas or liquid is filtered through the cartridge by passingfrom the high pressure or inlet side to the low pressure or outlet sideof the filter. The filter cartridge thus includes an inlet passage 312or passages for passing gas or fluid to the inlet side of the filterlayer to be filtered, and an outlet passage 314 or passages for passinggas or fluid that has been filtered through the filter layer 304. Filter3B shows an end view of the outlet side of the particular filtercartridge shown in FIG. 3A.

As noted, the filtration cartridge may be incorporated into largersystems depending upon the ultimate application. For example, airfilters may be incorporated into heating and air conditioning or otherenvironmental control systems to provide purified air for, e.g.,commercial or industrial, i.e., clean room, applications. Filtrationcartridges of the invention may optionally be incorporated into fluidfiltration systems as well, for water, fuel or chemical filtrationapplications.

In accordance with the filtration applications, effective pore sizes ofthe filter media may be varied depending upon application, e.g., fromcoarse particle filtration, e.g., effective pore sizes of 1, 10 or moremicrons, to antibacterial filtration, e.g., effective pore sizes of 0.2μm or less, e.g., down to 20 nm or less. As alluded to elsewhere herein,the phrase “effective pore size” does not necessarily reflect the sizeof a discrete passage through the substrate, but instead may reflect thecross sectional dimensions of a contiguous path through which fluid, gasor particles may pass, or be blocked from passing. In addition, the“effective pore size” of a given passage does not necessarily define theabsolute dimensions of the contiguous passage, but instead defines thesize of the particles that are effectively blocked from passing throughthe passage. Typically, such varied pore sizes will be of a function ofnanowire density disposed within the larger apertures that exist in theunderlying substrate, the diameter and length of the nanofibers, as wellas a result, to some extent, of the size of such apertures to beginwith.

Similarly, the composition or make up of the filtration media, both interms of nanofibers and the underlying substrate, may depend upon theapplication to which the material is to be put, with materials beinggenerally selected to withstand the conditions to which they will beexposed. Such conditions might include extremes of temperature,alkalinity or acidity, high salt content, etc.

In one aspect the invention includes methods of filtering a fluid orgas. In the methods, a porous substrate is provided and the gas orliquid is passed through the porous substrate to filter the gas orliquid. The substrate has a plurality of apertures disposed therethroughto provide a porous substrate that has an overall surface area thatincludes an interior wall surface area of the apertures, and thesubstrate comprises a plurality of nanowires attached to at least aportion of the overall surface area of the porous substrate.

2. Breathable Moisture Barriers

In a related aspect, the substrate is configured to be permeable to gas,e.g., air, while remaining impermeable to liquid. For example, suchbarriers are particularly useful as breathable moisture barriers forclothing and medical applications, allowing moisture vapor, oxygen andother gases to pass through the barrier freely, but preventing liquidfrom passing. In accordance with the invention, this is accomplished byproviding nanofibers within the apertures that are disposed through theporous substrate. In contrast to other aspects of the invention,however, the nanofibers for the moisture barrier applications areselected or treated to have increased hydrophobicity. Treatment ofnanofibers surfaces to increase hydrophobicity was described in detailin U.S. Patent Application Publication No. 20050181195, which is herebyincorporated herein by reference in its entirety for all purposes. Inparticular, the nanofibers and or the substrate surface may bederivatized to attach hydrophobic chemical moieties to their surfaces toincrease the hydrophobicity of the material. Those of ordinary skill inthe art are well versed in the coupling of hydrophobic chemical moietiesto substrates, including, e.g., silane chemistries for treating silicabased substrates, and the like. By providing such super hydrophobicnanofiber surfaces on porous underlying substrates, one can preventpassage of liquids, e.g., liquid water or other aqueous solutions, whilepermitting air, water vapor or other gases to pass. Typically, suchbarriers will be substantially impermeable to moisture, e.g., preventingpassage of the substantial majority of moisture that comes into contactwith the surface under ambient conditions.

Such moisture permeable barriers are particularly useful in outdoorgear, such as clothing, shelters, etc. where it is desirable toeliminate moisture generated from within, while not permitting liquidwater to enter. FIGS. 4A and 4B schematically illustrate a layeredtextile product, e.g., coat 400, that is comprised of a layered textile402 (shown in exploded view in FIG. 4B). As shown, a layer of the porousnanofiber bearing substrate material 408 is provided between layers ofother material, e.g., a nylon outer shell 404 and cotton orpolypropylene fabric lining 406, which provides external protection fromwind and cold and internal comfort against the skin or clothing of thewearer.

In one aspect the invention includes methods of producing a gaspermeable moisture barrier. In the methods, a porous substrate thatcomprises a plurality of apertures therethrough and a plurality ofnanofibers attached to at least a portion of an overall surface area ofthe porous substrate are provided. The porous substrate and nanofiberstogether provide a gas permeable barrier. At least the nanofibers aretreated to increase hydrophobicity of the nanofibers attached to theoverall surface of the porous substrate, to provide a gas permeablemoisture barrier.

3. Bandages

In another preferred embodiment, these moisture permeable barriers areuseful as bandages or wound dressings, as they allow oxygen to reachwound areas, gas and vapor to escape wound areas, all the whilepreventing liquid water and other harmful forces/abrasions, etc., fromcontacting the wound areas. In addition to their benefits assemi-permeable barriers, the nanofiber coated surfaces also may provideadhesion, to maintain the bandage in place, e.g., adhering the bandageto itself or the skin around the wound area. Use of nanofiber surfacesas dry adhesives or high friction materials is described in detail inU.S. Pat. No. 7,056,409, and incorporated herein by reference in itsentirety for all purposes. Additionally, such nanofiber coatings maycomprise antimicrobial materials, e.g., ZnO or the like, to help preventany infections in the wound areas.

FIG. 5 schematically illustrates a self adhesive, semi permeable,moisture repellant bandage as described above. As shown, a bandage 500includes a flexible porous substrate strip 502 of the invention, e.g., awoven fabric or soft mesh material, i.e., a polymer or cloth mesh,having nanofibers that are appropriately treated to provide ahydrophobic barrier, e.g., a moisture barrier (represented by hatchingon strip 502). The substrate strip 502 functions both as a breathablemoisture impermeable cover and as an adhesive strip. A protective pad504 is provided upon a portion of one side of the substrate strip 502 toprovide protection for a wound that is covered by the bandage. Whenapplied to a wound, the protective pad overlays the wound to provideprotection from rubbing or other contact, while the portions 506 and 508of the strip provide adhesion to the surface tissues adjacent a woundedarea (or when wrapped completely around a wounded appendage, to anopposing surface of the other end of the substrate strip, e.g., region506 adheres to the back side of region 508).

B. Vapor Barriers

In a related aspect, the porous substrates of the invention are usefulin breathable chemical/biological protective clothing and apparatus, toadsorb or decompose various organic and inorganic vapors. In accordancewith the vapor barrier applications of the present invention, poroussubstrates (e.g., fabrics or flexible meshes) are used as thefoundation. Preferably, an activated carbon fabric is used. Theactivated carbon fabric absorbs organic vapors and acts as a supportstructure. Activated carbon fabric is commercially available, e.g., fromSpectracorp.

Nanofibers (e.g., silicon nanowires, carbon nanotubes, or polymericnanofibers) are embedded, disposed on, or grown in situ on the activatedcarbon fabric or other substrate. The nanofibers reduce the permeabilityof the activated carbon fabric such that adsorption of vapors is moreefficient, permitting the layer to be thinner. A hydrophilic surface isoptionally present on the nanofibers (e.g., due to the nanofibercomposition or to surface treatment of the nanofibers with a hydrophilicmaterial), such that a high degree of moisture vapor transmission can bemaintained through the layer despite the reduced air permeability.

The nanofibers are functionalized with moieties that bind or decomposevapors. For example, the nanofibers can be functionalized with moietiesthat bind or decompose non-organic gases such as phosgene which are notabsorbed by activated carbon. The nanofibers can be functionalized witha chemical moiety, e.g., a chemical moiety that absorbs or decomposes anon-organic gas, such as a carboxylic acid moiety which binds ammonia ora moiety selected from Table 1. In a preferred aspect, the nanofibersare functionalized by attachment of nanocrystals to the nanofibers. Thenanocrystals can comprise a material such as Ag, ZnO, CuO, Cu₂O, Al₂O₃,TiO₂, MgO, FeO, MnO₂, Zn, or a material selected from Table 1, forexample. Multiple functionality can be conveniently imparted by usingnanocrystals of different compositions, attaching two or morepopulations of nanocrystals or other nanostructures comprising differentmaterials adsorbing or binding different gases to the nanofibers (e.g.,two, three, four, five, six, or more populations). In a related aspect,two or more batches of nanofibers can be chemically derivatized, mixed,and incorporated into the fabric. Similarly, two or more populations ofnanofibers can be synthesized from different materials, e.g., selectedfrom Table 1, mixed, and incorporated into the fabric (with or withoutadditional functionalization), or nanofiber longitudinalheterostructures (where a single nanofiber comprises two or morematerials) can be used. In a related aspect, nanocrystals or othernanostructures can be attached to a mesoporous carbon fabric or otherfabric, particularly another high surface area fabric, even in theabsence of nanofibers.

A nanocrystal generally denotes a structure that is substantiallymonocrystalline (or the core of which is substantially monocrystalline).Nanocrystals typically have a diameter that is less than 500 nm andpreferably less than 100 nm or 50 nm and in many cases, less than 20 nmor 15 nm. Nanocrystals optionally have an aspect ratio of less than 10,for example, less than 5 or 2, and in some cases, between about 0.1 andabout 1.5. Exemplary nanocrystals include, but are not limited to,substantially spherical nanocrystals (e.g., spherical nanocrystalshaving a diameter between 1 nm and 6 nm), rod shaped crystals (e.g.,rods that are about 5×50 nm), and tetrapods (also called nanotetrapods,which have four rod-like arms coming out of a central core). Techniquesfor chemically modifying nanofibers and/or attaching nanocrystals thenanofibers are well known in the art; for example, coating thenanofibers with polylysine or a polymer. Attachment can be covalent ornoncovalent.

FIG. 13 schematically illustrates a nanostructure-enhanced fabric asdescribed above. As shown, woven fabric 1301 includes apertures 1302.Nanofibers 1304 are attached to the surface of the fabric, andnanocrystals 1303 are attached to the nanofibers.

The fabric or other substrate can be incorporated into an article ofclothing, for example, a protective suit, or into other protectiveapparatus. The nanofiber-enhanced fabric is optionally protected byapplication of a layer of porous fabric or other porous material on oneor both sides.

The vapor-absorbing fabrics of the invention offer a number ofadvantages over current protective layers. For example,chemical/biohazard protection suits for the U.S. military are currentlybased on activated carbon pellets embedded in a urethane polymer.Although the pellets absorb many hazardous vapors, the suits have a lowmoisture vapor transmission rate and are susceptible to many non-organicvapors. Due to its extremely high surface area and high binding affinityfor organics, activated carbon cloth has been used commercially inhazardous protection suits. However, the weakness of these fabrics istheir inability to absorb non-organic molecules. By filling in theinterstices of the fabric with nanofibers that are functionalized todecompose or absorb the non-organic vapors, a single layer fabric suchas those described above can provide full functionality whilemaintaining good moisture vapor transmission.

TABLE 1 Exemplary materials for functionalization of nanofibers. ToxicIndustrial Compounds Absorber/Decomposer Chemical Inorganic Acid NameCompound form Warfare Agents Gasses Organic Biological Aluminum OxideAl₂O₃ Ammonia, VX, SO₂ 4-Vinylpyridine, p- GD, HD, Cresol Simulants:2CEES, DMMP Titanium Dioxide TiO₂ Ammonia, HCl, Formaldehyde, VX, GD,HD, Nicotine, p-Cresol Simulants: 2CEES, DMMP, Paraoxon Magnesium OxideMgO Ammonia, HCl, SO₂, CO₂ Acetaldehyde, 4- Antimicrobial/biocidal VX,GD, HD, Vinylpyridine, (MgO*I₂, MgO*Cl₂) Simulants: Formaldehyde, DMMP,Paraoxon Nicotine Copper Oxide, Cu Salts Cu₂O, CuO, CuSO₄*5H₂O, Ammonia,H₂S, SO₂, NO₂ CuCl₂*2H₂O, Phosgene, HCN, Cu(NO₃)₂*3H₂O Chlorine, ArsineZinc Oxide, Salts ZnO, Na₂ZnO₂, Paraoxon, H₂S Antimicrobial/biocidalAmmonia, Phosgene, HCN, Arsine Silver, Silver Oxide, Ag Salts Ag,Maglon, Ag₂O, Arsine, Antimicrobial/biocidal Surfacine, AgNO₃ Phosphine,Phosgene, Chlorine, Diphosgene Iron Oxides FeO, Fe₂O₃ H₂S Mercaptans(CH₃SH), Dimethyl Sulfide, Halogenated Hydrocarbons Manganese Oxide,Salt MnO₂, Mn_(x)O_(y), KMnO₄ CO (w/CuO), Aldehydes NO₂, H₂S ChromiumSalts CuCrO₄, Cyanogen NO₂, H₂S CuCrO₄*NH₃*5H₂O, Chloride (ClCN)Na₂Cr₂O₇, (NH₄)2Cr₂O₇ Polyoxometallates H5PV2Mo10O40 type GD, HD (POMs)heteropoly acids, PW9O37 Sulfur Mercury Potassium Salts K₂CO₃, KI,K₂MnO₄ Phosphine, SO₂, NO₂, H₂S, Methyl Iodide Arsine Carbon Disulfide,(radioactive) Mercury Phosphoric & Sulfuric acid H₃PO₄, H₂SO₄ AmmoniaMercury Amine, Acetaldehyde Pyridine C₅H₅N Cyanogen ChlorideTriethylenediamine Methyl Iodide (TEDA) (radioactive) Para-Aminobenzoicacid H₂S (PABA) Ortho-Iodosobenzoic acid GB, GD, GA, (IBA) simulantDimebu

C. High Contact Surface for, e.g. Electrical Interfacing

As alluded to previously, the above described applications typicallyemploy nanofibers disposed upon a porous substrate to provide enhancedproperties to the porous substrates, e.g., enhanced porosity forfiltration, moisture repulsion, etc. However, in a number ofapplications, applying nanofibers to a porous substrate provides aunique material that is not employed simply for its porosity, but forother properties that are enhanced by the synergistic structuralcharacteristics of small dimension materials coupled to a high surfacearea underlying substrate.

In particular, application of nanofibers to a porous substrate, as aresult of its higher surface area, provides for higher packing levels ofnanofibers per square centimeter of projected area. In particular, densemats of nanofibers may be provided joined together on the poroussubstrate, which density levels would not be readily achieved on flatsurfaces. The higher surface area of nanofibers is also readilyaccessible via the apertures or pores in the underlying substrate.

In addition to the increase in nanofiber densities and/or higher surfaceareas, porous substrates, e.g., meshes and fibrous mats, tend to beflexible by comparison to more rigid, solid substrates, e.g., siliconwafers, metal plates, or the like. Additionally, depending upon therelative porosity of the substrate, the overall article may benefit frombeing partially or even substantially translucent or transparent, e.g.,like a window screen.

D. High Surface Area/High Density Fiber Applications

Separate and apart from the properties set forth above, poroussubstrates can also provide a lightweight, high density lattice formaintaining, handling, storing and otherwise using nanofibers.Nanofibers may be harvested from this lattice, or portions of thelattice may be used in their entirety to be applied in more nanofiberspecific applications, e.g., as semiconductive elements, compositefiller materials for structural or electrical enhancement, high surfacearea matrices, e.g., for separations, or the like.

In still other applications, the porous substrates having nanowiresdisposed thereon may provide electrical integratability to thenanofibers (or in this case, specifically nanowires) that are attachedthereto. Specifically, use of conductive porous substrates may provideat least a portion of the electrical connection to the nanowiresnecessary for the given application. For example, semiconductornanowires coupled to a metallic or other conductive or semiconductivemesh are already partially integrated into an electrical circuit, e.g.,the mesh becomes an electrode, e.g., source or drain, in the overalldevice.

The following description includes a number of such specific examples ofapplications that benefit from the aforementioned properties forillustration purposes alone. However, a much larger number of specificuses and applications of the substrates and articles of the inventionwill be readily apparent to those of skill in the art upon therealization of the above-described benefits, and the followingdescription should not be viewed as limiting and in no way excludes suchapplications.

In a first exemplary application, the substrate of the invention is usedas one electrode in a diode configuration. In particular, a conductivemesh is used as the underlying porous substrate with nanowires attachedto its surfaces, e.g., overall surface. The composition of the mesh isselected to have a work function that promotes conduction of the majorcarriers in the nanowire portion. For its part, the nanowire portion isselected to provide one half of the diode circuit, and may include,e.g., p doped nanowires. In accordance with this architecture, thenanowire coated porous substrate functions as a portion of the diodecircuit. The other portion of the diode circuit may be provided aseither a conventional semiconductor substrate or as a mirror image ofthe first, except with the materials being selected to conduct the othercarriers, e.g., holes, e.g., by providing n-doped nanowires andappropriate electrode compositions for the underlying substrate. The twosubstrates are then mated to interface the nanowires at the surface toprovide the functioning diode. Additional elements may also be providedto ensure proper contact between the nanowires, including conductiveelements, annealing steps, etc.

In another exemplary application, high surface area nanowire coatedsubstrates may be used as electrodes for interfacing with otherelements, e.g., electrical or non-electrical, such as human tissue forelectrical stimulation of the tissue. By way of example, electrodes forpace makers typically benefit from having high surface areas, and thusmaking more complete contact to the tissue they are stimulating.Relatedly, where the nanofiber coated article is being used as a tissuelattice, e.g., to facilitate bioincorporation, higher surface areas andgreater porosity are highly beneficial in providing adherence pointswithout blocking access to such tissue by nutrients etc. Specifically,as described in U.S. Patent Application Publication 20060159916,nanofiber coated surfaces on medical implants provide ‘non-tortuouspath’ enhanced surface areas that can provide enhanced tissue adhesionand bioincorporation. It is expected that by providing such nanofibersurfaces over porous underlying substrates, these properties will befurther enhanced.

In a particular application, such a diode arrangement is employed as aphotoactive element, e.g., as a photovoltaic or photodiode device. Theuse of partially or substantially translucent porous substratesfacilitates this application in letting light pass through the electrodecomponents to impinge on the semiconductor nanowires, thus generatingcharge separation at the heterojunction of the opposing nanowires.Selection of materials for the opposing underlying substrates may followthe same criteria as used in conventional photovoltaic devices. Forexample, one underlying substrate mesh may be comprised of aluminumwhile the other is comprised of another metal having a different workfunction, e.g., ITO, or a similar conductive material.

Previously described nanocomposite photovoltaics have employed an activelayer of a nanocomposite material sandwiched between two conductivelayers that function as electrodes. The upper electrode typicallycomprises a transparent conductive coating on the active layer, e.g.,indium tin oxide (ITO). These nanocomposite photovoltaic devicesemployed a first component in which initial charge separation occurs.This typically employed a nanocrystal in which an exciton was createdupon exposure to light. This nanocrystal component typically conductsone charge carrier better than the other, e.g., electrons. Thenanocrystals are typically disposed in a matrix of another materialwhich conducts the other charge carrier, e.g., holes, away from thenanocrystal component. By conducting the two carriers to oppositeelectrodes, one generates an electric potential. Typically, the holeconducting component comprises an organic semiconducting polymer, e.g.,poly-3-hexylthiophene (P3HT), although the hole conducting component canbe another nanocrystal of a different composition. The overallarchitecture of a nanocomposite photovoltaic device is described indetail in, e.g., U.S. Pat. No. 6,878,871.

In accordance with the present invention, the overall photovoltaicdevice 600 includes one (as shown in FIG. 6A), or two (as shown in FIG.6B) porous substrates 602 and 604 upon which semiconducting nanowiresare deposited. The first porous substrate 602 typically comprises afirst conductive mesh 602 a or other porous material (as describedpreviously herein) that functions as one electrode in the system, e.g.,the lower electrode, and includes a first population of nanowires 606 ofa first composition attached to its overall surfaces. In a firstembodiment shown in FIG. 6A, the first porous substrate 602 and itsassociated nanofibers are coated with a conductive matrix material 608that has a type-II band gap offset from the nanowire population 606, soas to affect charge separation. A transparent electrode 610 is thenprovided over the matrix layer 608.

In a second exemplary embodiment, the upper, transparent electrode 610in FIG. 6A is replaced by the second porous substrate 604, which isagain fabricated from a conductive mesh 604 a, but which includes adifferent work function from that of the first porous substrate 602, tofacilitate charge separation. A second population of nanowires 612 isprovided attached to the second porous substrate 604. The composition ofthe first and second nanowire populations 606 and 612, respectively, isselected to provide a type-II bandgap energy offset, again, so as tofacilitate charge separation and differential conduction. The first andsecond porous substrates (602 and 604) are then mated together such thattheir respective nanowire populations 606 and 612, respectively, are inelectrical communication so as to permit charge separation between thetwo layers. As noted elsewhere, herein, in some cases, the opposingnanowire populations may be further processed to permit such electricalcommunication, including, e.g., thermal annealing, or the like. The useof a dual semiconductor system, e.g., as shown in FIG. 6B, may obviatethe need for any organic species within the active layer, e.g.,conductive polymers, or the like, and is expected to improve chargeseparation efficiencies by speeding conduction of their respectivecarriers to their respective electrodes, and thus prevent recombinationof the charges within the active layer.

In one embodiment, an electrical device of the invention includes aporous substrate having a plurality of apertures disposed therethrough,the porous substrate having an overall surface area that includes aninterior wall surface area of the plurality of apertures, and aplurality of conductive or semiconductive nanowires attached to andelectrically coupled to the porous substrate. An exemplary photovoltaicdevice includes a first such electrical device, wherein the plurality ofnanowires comprises a first energy band gap, and a second suchelectrical device, wherein the plurality of nanowires comprises a secondenergy band gap. The first and second energy band gaps display a type-IIband gap offset relative to each other, and the nanowires of the firstelectrical device are in electrical communication with the nanowires ofthe second electrical device, so as to allow charge separation betweenthe first and second electrical devices upon exposure to light.

As will be readily appreciated, the photovoltaic devices described aboveare primarily for the illustration of the applicability of thesubstrates of the invention to certain electronic or optoelectronicapplications. Those of skill in the art will recognize a broad range ofother electronic devices in which such substrates would be useful.

In still another exemplary application, porous nanofiber or nanowirecoated substrates are encased in matrix components, e.g., a polymermatrix, for use as a composite matrix, including the underlying mesh.Such applications are particularly useful where the nanofibers are beingemployed as a bulk material to enhance the functionality of thecomposite matrix. Such enhancements include electrical enhancements,e.g., where the composite is being used as a dielectric material, or topartially orient the nanofibers in optoelectric applications, e.g.,photovoltaics, structural enhancements where the presence of thenanofibers imparts unique structural characteristics to the matrix,e.g., tensile strength, elasticity etc.

FIG. 7 schematically illustrates a composite matrix incorporating thematerials of the invention. As shown, a film of composite material 702includes within a matrix material, e.g., a polymer, ceramic, glass orthe like, a porous substrate 706 that includes nanofibers 708 disposedupon its surface 710, including within pores or apertures 710. Theporous substrate is generally immersed or impregnated with matrixmaterial 704 to provide film 702. As noted above, these composite filmsare then applied in a variety of applications, e.g., as conductivefilms, dielectric films, etc.

FIGS. 8A and 8B schematically illustrate the use of porous substrates toprovide high surface area matrices for separation applications, e.g.,chromatography. In particular, as shown in FIG. 8A, a porous substrate802 that has nanofibers attached to its surface is provided. As noted,this porous substrate may be provided in a number of different forms.For example, substrate 802 may comprise a mesh or screen that is rolledinto a cylinder, either before or after the fibers are attached or grownupon it. Alternatively, the substrate may comprise a solid, but sinteredfritted material, e.g., metal or glass. In still other aspects, thesubstrate may comprise a fibrous material, e.g., glass wool, wovenfabric etc., that is shaped into the desired shape, e.g., a cylinder 802as shown, either by forming the material as such or packing the materialinto a cylindrical (or other shaped) housing. Again, such shaping maytake place either before or after the nanofibers have been grown orotherwise attached to the surface of the porous material.

The substrate 802 is then placed into a column 804, which includes aninlet 806 and an outlet 808 through which fluids are flowed into and outof the column during a separation operation. As shown in FIG. 8B thecolumn 804 is then connected to appropriate liquid handling equipment,e.g., gradient makers 810, pumps 812, detectors 814, fraction collectors816, and the like, for carrying out chromatographic separations.

As will be apparent, the separation matrices incorporating the substratematerials of the invention may encompass any of a variety of thedifferent substrate structures and conformations, employ any number of avariety of different types of nanofibers, as described elsewhere herein.Such structures, conformations and compositions are generally selecteddepending upon the particular application to which they are to be putand which will generally be appreciated by those of ordinary skill inthe art.

E. Reinforcing Lattice for Composite Materials

In still another aspect of the invention, the porous, nanofiber bearingsubstrates of the invention form the lattice of a composite material toenhance the integration of the lattice and improve the structuralcharacteristics of the overall composite material. In particular, anumber of composite materials include a lattice that provides theunderlying structural integrity that supports an additional material,e.g., epoxies or other polymers, ceramics, glasses, etc. For example,composites of fiberglass cloth encased in epoxy resins, or otherpolymers are routinely used in a variety of different applications,including, e.g., furniture, surfboards and other sporting goods, autobody repairs, and the like. Likewise, carbon fiber cloths or substratesare also generally encased in a polymer or epoxy resin before they areformed into the desired shape. Ultimately, these composite materialsgenerally possess structural characteristics, e.g., strength to weightratios, that are better than most other materials. Without being boundto a particular theory of operation, it is believed that the interactionof the encasing material and the lattice material is of significantimportance in these structural characteristics. Specifically, it isbelieved that by enhancing the interaction of the two components of thecomposite, e.g., improving integration of one into the other, willimprove the strength of the ultimate composite material. Because thenanofiber bearing porous substrates of the invention benefit fromextremely high surface areas, as compared to that of the poroussubstrate alone, it is expected that they will possess substantiallygreater interactivity with the surrounding encasing material, e.g., theepoxy. As such, another aspect of the invention includes the use of theporous substrates having nanofibers deposited thereon, as a latticematerial for a composite material.

A general illustration of this aspect of the invention is shown in FIG.11. As shown, a porous substrate 1100 having a surface that includesnanofibers 1102, is immersed within a matrix material, e.g., hardenedpolymer 1104 to provide a composite material 1106, that may befabricated into a variety of different materials or articles ofmanufacture.

As noted, a variety of fabrics are generally incorporated into compositematrices as a supporting lattice for the ultimate material. For example,carbon fiber composites typically employ a woven carbon fiber materialwhich is then intercalated with a resin, e.g., an epoxy or otherpolymeric material. The composite material is then formed into a desiredshape and allowed to cure. Alternatively, the desired shape may beformed post curing, e.g., by sanding or otherwise sculpting the hardenedmaterial. Similarly, woven glass fabrics are used in fiberglasscomposite materials by intercalating the fabric with an appropriatematrix, e.g., an epoxy, etc.

In the context of the present invention, a porous substrate that hasnanofibers deposited upon it or attached to its surface(s) is used asthe lattice material for the resultant composite. The nanofiber materialis intercalated with a matrix material that substantially or at leastpartially fills the voids within the material. Because of the extremelyhigh surface area, the matrix binds to and integrates the latticematerial extremely well, resulting in a stronger composite material thanthose based simply on a porous substrate alone, e.g., in the absence ofthe nanofiber surfaces. Such composite materials may be generallyemployed in a number of applications where high strength to weightratios are desired, such as in lightweight engineered parts, e.g.,bicycles, tennis rackets, automotive parts, aviation parts, satelliteand other extraterrestrial equipment and parts, etc.

While virtually any porous substrate material, e.g., as describedelsewhere herein, may be employed as the supporting lattice, for anumber of applications, a flexible lattice material is more desirable,as it may be later conformed to a desired shape, e.g., molded orsculpted, for a particular application. In at least a first preferredaspect, flexible mesh materials are used as the supporting lattice. Suchmaterials include porous polymeric sheets, porous metal sheets, flexibleporous glass sheets, e.g., sintered glass sheets, and the like. In otherpreferred aspects, porous woven cloth-like materials are employed as thelattice, including, e.g., woven polymeric fabrics, (e.g., polyesters,nylons, polyetherketones, polyaramid, etc.), woven glass fabrics (e.g.,fiberglass fabrics, glass wool, etc), carbon or graphite fiber fabrics,Kevlar fabrics, and metallic fiber fabrics (e.g., titanium, stainlesssteel, nickel, platinum, gold, etc.). The wide range of differentporous, flexible substrates for use as the lattice material willgenerally be appreciated by those of ordinary skill in the art, and maygenerally be varied to accomplish the needs of the ultimate application,e.g., light weight and/or enhanced strength, materials compatibility,and the like.

Like the lattice material, the type of material used as theintercalating matrix for the lattice will generally depend upon thenature of the application to which the material is to be put. By way ofexample, inorganic materials may be employed as the matrix, includingglass, ceramics or the like. Alternatively, and preferably, polymericmatrices are employed, including thermosets, such as polyester, epoxy,urethane and acrylate resins, and the like, thermoplastics and/orthermoplastic elastomers, such as polyethylene, polypropylene, nylon,PFA, and the like. Typically any of these matrix materials may bedeposited as a polymer over the lattice substrate and allowed tointercalate throughout the nanofiber mesh. Subsequently, the matrixmaterial is allowed to or caused to cure in situ. Alternatively,polymeric matrices may be intercalated as a monomeric solution andpolymerized in situ to “cure” the matrix in place. In still furtheralternate aspects, the polymeric matrix may be deposited over the poroussubstrate bearing the nanofibers, using a vapor phase or solventdeposition process, e.g., as described above for the cross-linking ofnanofibrous mats. The full range of different polymers and their utilityin a wide range of different applications will be readily apparent tothose of ordinary skill in the art.

F. Protected Nanofiber Surfaces

A variety of applications exist for articles with nanofiber-enhancedsurfaces, as noted above; for example, implantable medical devices suchas stents, nanofiber-enhanced fabrics, nanofiber arrays, membranes, andthe like. However, such nanofiber-based devices frequently present arelatively fragile nanofiber surface at which even light contact withother objects (e.g., packaging material, skin, etc.) can cause nanofiberbreakage, matting, and removal. Techniques for strengthening and/orprotecting nanofiber surfaces are thus desirable.

As described above, nanofibrous mats can be fused, coated, orcrosslinked at points of nanofiber contact. Similar techniques can beused to protect essentially any population of nanofibers, whether grownin situ or deposited on a substrate, that would benefit from protectionfrom abrasion, breakage, etc.

In one aspect, the invention provides methods of stabilizing nanofibers(e.g., nanowires). In the methods, a population of nanofibers isprovided, and a coating is formed on the nanofibers. The coating iscontiguous between adjacent nanofibers in the population.

In one class of embodiments, a first material comprising the nanofibersis different from a second material comprising the coating. The firstand second materials are optionally unrelated or related. Thus, incertain embodiments, the second material is an oxide of the firstmaterial. For example, the nanofibers can comprise silicon and thecoating silicon oxide, the nanofibers can comprise titanium and thecoating titanium oxide, etc. Generally, regardless of the composition ofthe nanofibers, the coating optionally comprises an oxide, e.g., anoxide of silicon, titanium, aluminum, magnesium, iron, tungsten,tantalum, iridium, or ruthenium. For example, titanium nanowires can beoxidized to form the titanium oxide coating, or other nanowires can becoated with titanium which is then oxidized to form the coating. Thenanofibers are optionally sintered or oxidized in situ during synthesis,or after synthesis and/or deposition by a rapid thermal oxidation (RTO)technique, e.g., in embodiments in which any substrate to which thenanofibers are attached is compatible with the high temperaturesrequired for RTO. As another example, as described above, siliconnanowires (nanofibers) can be synthesized (e.g., at about 480° C.) andthen coated with polysilicon (e.g., at about 600° C.) to thicken andstrengthen the nanowires and to fuse wires together at wire-wirejunctions.

It will be evident that the coating is not limited to comprising anoxide or polysilicon, but can include essentially any material thatimparts a desirable property to the resulting coated nanofiberpopulation, e.g., stability or a desirable electrochemical or dielectricproperty. Additional exemplary coatings include polymers, carbon,carbides, and nitrides. For example, silicon nanowires can besynthesized and then coated with carbon and silicon carbide (resulting,e.g., from conversion of some of the silicon nanowire) or with TaN(e.g., by atomic layer deposition).

In one aspect, the population of nanofibers is provided by synthesizingthe nanofibers on a surface of a substrate. Exemplary substrates havebeen described herein, e.g., porous, curved, woven, and/or flexiblesubstrates, but it will be evident that the methods are not limited tonanofibers synthesized on or attached to such substrates. The nanofiberscan be preformed and deposited on a substrate. The substrate isoptionally non-porous. The substrate optionally comprises or covers atleast a portion of a surface of an implantable medical device.

The coated population of nanofibers preferably retains any desirablesurface properties of the original nanofiber population, for example,minimal contact area with objects, a highly porous three-dimensionalstructure, superhydrophobicity, low biological growth and attachment, orthe like. Relatedly, the coating can provide desirable surfacecharacteristics; for example, the coating can comprise a material thatis readily functionalized (e.g., silicon oxide). In one class ofembodiments, the methods include functionalizing the coating with achemical binding moiety, a hydrophobic chemical moiety, a hydrophilicchemical moiety, a drug (e.g., to inhibit cell or bacterial growth), orthe like.

Populations of nanofibers (e.g., nanowires) formed by the methods areanother feature of the invention. One general class of embodimentsprovides a population of nanofibers that includes nanofibers and acoating on the nanofibers, wherein the coating is contiguous betweenadjacent nanofibers in the population. A device bearing such apopulation, e.g., an implantable medical device, is likewise a featureof the invention.

In a related aspect, a nanofiber surface is protected by a layer ofporous (or alternatively, non-porous) material. A nanofiber bearingsubstrate can have a first layer of porous material disposed on itssurface. For example, a nanowire coated fabric, flexible mesh, or otherflexible and/or porous substrate can be protected with a porous materiallayer that can, e.g., be heat sealed or ultrasonically welded to thesubstrate, on one or both sides of the substrate. This protects thedelicate nanofiber bearing substrate from abrasion or similar damage,while still allowing vapors and/or liquids to penetrate to thesubstrate. Exemplary porous materials include, but are not limited to,non-woven polypropylene porous material (for example, similar to thatused in Celgard™ laminates, Hoechst Celanese). Additional weldablematerials include polyethylene, polystyrene, acetate, and thinpretreated Teflon™ layers. The protective layer(s) are optionallyflexible or inflexible.

The substrate is optionally sandwiched between two layers of porous ornon-porous material; for example, between two layers of porous material,or between a porous layer on one side and a non-porous layer on theother side. It will be evident that choice of material(s) for theprotective layer(s) can depend on the desired application. For example,a flexible fabric substrate can be sandwiched between flexible porouslayers for certain applications, or between a porous layer and aninflexible non-porous layer for other applications. As noted, aprotective layer can be heat sealed or welded to the substrate.Similarly, the protective layer can be attached to the substrate bysewing or use of an adhesive.

FIG. 14 schematically illustrates protection of a nanofiber bearingsubstrate by a porous layer. As shown in FIG. 14, substrate 1402 bearingnanofibers 1403 is provided. The substrate is typically a flexibleporous substrate, but need not be; in certain embodiments, it is a rigidand/or non-porous substrate. First layer 1404 of porous material isdisposed on the first surface of the substrate and second layer 1405 ofporous material is disposed on the second surface of the substrate,providing article 1401. Nanofibers 1403 are protected from abrasion,matting, removal, breakage, etc. by being sandwiched between the porousmaterial layers.

G. Nanofiber Synthesis

In one aspect, the porous substrates of the invention are useful aslattices for synthesis of large quantities and/or high densities oflong, unbranched nanofibers, particularly nanowires. Silicon nanowires,for example, are desirable materials for inter alia a variety ofmacroelectronic applications. Such nanowires are typically required tobe long (e.g., at least about 40, 50, or 60 μm in length), straight, andbranch free. However, large quantities of such long and unbranchednanowires are not readily obtained through current synthesis techniques

Currently, nanowires are typically grown on flat wafer substrates. Whennon-oriented wire growth methods (e.g., SiH₄), which are simpler andless expensive to implement than oriented growth methods, are used toproduce nanowires, collisions between growing wires lead to growthtermination, branched wire formation, and the like. Collisions betweennanowires can be limited by reducing nanowire density (e.g., by reducinggold colloid particle density) and/or length; however, these tacticsobviously do not result in inexpensive, simple production of largequantities of nanowires and/or long nanowires. For example, fornon-oriented growth on a flat substrate, to obtain 40 μm long nanowireswith only 10% collisions per wire, gold colloid particle density can beat most 0.01 particle/μm², based on both theoretical predictions andexperimental results. Yield of long, straight, unbranched nanowires fromeven non-oriented growth techniques can be improved by growing thenanowires on high surface area porous substrates or curved substrates.

In other embodiments, nanofibers are grown on porous surfaces to providethe semipermeable membranes and other articles noted above, for example,and need not be long, straight, and/or unbranched.

Accordingly, one general class of embodiments provides methods ofproducing nanofibers. In the methods, a substrate comprising a) aplurality of apertures disposed therethrough, the substrate comprisingan overall surface area that includes an interior wall surface area ofthe plurality of apertures, or b) a curved surface is provided. Aplurality of nanofibers is synthesized on the substrate, wherein theresulting nanofibers are attached to at least a portion of the overallsurface area of the substrate of a) or to at least a portion of thecurved surface of b). The curved surface is preferably convex when usedfor growth of long unbranched nanofibers, but can alternatively oradditionally be concave. The curved surface optionally has a nonzeromean radius of curvature over a significant fraction of the substrate'ssurface (e.g., a cylindrical fiber-shaped substrate) or over its entiresurface (e.g., a microsphere or similar substrate). It will be evidentthat many substrates can be described as either or both porous and/orcurved; for example, a fibrous mat can be described as a whole as aporous substrate or on the level of individual constituent fibers as acurved substrate.

A number of exemplary substrates have been described above. For example,the substrate can comprise a solid substrate with a plurality of poresdisposed through it, a mesh, a woven fabric, or a fibrous mat. As otherexamples, the substrate can comprise a plurality of microspheres orother microparticles or nanoparticles (e.g., glass or quartzmicrospheres or nanospheres), a powder (e.g., carbon black powder), aplurality of glass or quartz fibers (e.g., microfibers, fiberglass,glass or quartz fiber filters), or a foam (e.g., a polymer or metal foamsuch as reticulated aluminum). FIG. 15A shows reticulated aluminum,while FIG. 15B illustrates nanowires grown on a reticulated aluminumsubstrate. FIG. 18 Panel A depicts an electron micrograph of carbonblack powder having gold catalyst colloid particles deposited thereon.FIG. 18 Panels B and C depict an electron micrograph of siliconnanowires grown from the carbon black supported gold catalyst colloid ofFIG. 18 Panel A. In this particular method of growing silicon nanowires,the nanowires are grown directly from the catalyst supported carbonblack powder using a colloidal catalyst based VLS (vapor-liquid-solid)synthesis method such as those described above. In accordance with thissynthesis technique, the colloidal catalyst e.g., gold) is firstdeposited upon the surface of the carbon black powder. The carbon blackincluding the colloidal catalyst is then subjected to the synthesisprocess which generates nanofibers (e.g., nanowires) attached to thesurface of the carbon black particles. Typically, catalysts comprisemetals, e.g., gold, platinum, and the like, and may be deposited fromsolution onto the surface of the carbon black powder which is treated(e.g., oxidized) so that the catalyst colloids adhere to the surface ofthe carbon black powder. Other surface treatments include those thathave been described in detail previously, e.g., polylysine treatment,etc. The catalyst colloids may also be deposited in any of a number ofother well known metal deposition techniques, e.g., sputtering etc.Nanowires are then prepared by feeding, either by gravity or gasinjection (e.g., using an inert gas), the colloid-containing carbonblack particles and a reactive gas such as silane (SiH4) in a verticaltube reactor at about 480 degrees Celsius. The reactor includes a quartztube equipped with an internal quartz wool plug for receiving the carbonblack particles and a thermocouple for monitoring the reactortemperature. Inlet ports through which the catalyst, reactant gas, andpurge gas, e.g., argon, are added are also provided, as well as anoutlet port for venting the reactor. Following growth of the nanowiresfrom the carbon black supported metal colloid, the nanowires mayoptionally be heated in the quartz reactor to a temperature greater thanabout 500 degrees Celsius, e.g., between about 500 and 700 degreesCelsius, to evaporate the carbon black powder, thereby leavingfree-standing, detached nanowires. The nanowires are then harvested fromthe tube and can be filtered to remove any wires of poor quality andprepared for further manipulation and processing (e.g., integration intofunctional devices such as the membrane electrode assembly of a fuelcell or for other catalyst applications).

Essentially any other porous or curved substrate can also be employed inthe methods; preferred substrates are chemically compatible with anychemicals used to synthesize the nanofibers (e.g., have low levels ofmagnesium), can withstand the synthesis temperatures, can have catalystdispersed on them (if required), and facilitate clean harvest ofnanofibers from the substrate (if desired). In certain embodiments,e.g., for various filtration applications noted above, the poroussubstrate preferably has an effective pore size of less than 10 μm, lessthan 1 μm, less than 0.5 μM, or even less than 0.2 μm. In otherembodiments, e.g., for synthesis of long nanowires, the porous substratepreferably has an effective pore size of at least 25 μm, at least 50 μm,at least 100 μm, or more, depending on the desired nanowire length (forexample, the width of the apertures in a mesh used for nanowiresynthesis would be at least about twice the desired length of thenanowires).

The nanofibers can comprise essentially any type of nanofibers, e.g.,silicon nanowires, carbon nanotubes, or any of the other nanofibersnoted above. In certain embodiments, the nanofibers comprise nanowires,and the methods include synthesizing the plurality of nanowires bydepositing a gold colloid on at least a portion of the overall surfacearea of the substrate of a) or on at least a portion of the curvedsurface of b) and growing the nanowires from the gold colloid with a VLSsynthesis technique.

The methods optionally include surrounding or at least partiallyencapsulating the substrate and the resulting attached nanofibers with amatrix material; dissolving a soluble substrate following synthesis ofthe nanofibers on the substrate; forming a coating on the resultingnanofibers, wherein the coating is contiguous between adjacentnanofibers; disposing a layer of porous material on the resultingnanofibers (and optionally disposing the substrate on a second layer ofporous material, sandwiching the nanofiber-bearing substrate); and/orfunctionalizing the nanofibers (e.g., by attaching a chemical moiety ornanocrystal), as described above.

FIG. 16 shows electron and optical micrographs that illustrate certainaspects of the invention. FIG. 16 E shows a single glass fiber coveredby silicon nanowires. The glass fiber substrate was chemically treatedby soaking in a polylysine solution for 20 minutes to attract goldcolloid to its surface. Silicon nanowires were grown from the goldcolloid on the substrate using a chemical vapor deposition (CVD) method;a silane reaction at a moderately low temperature (480° C., much lowerthan the high temperatures of approximately 900° C. or more used by theindustry to grow bulk single crystalline silicon) resulted in singlecrystalline silicon nanowires. Quartz fiber filters have also been usedas substrates for growth of silicon nanowires. Two quartz fiber filters(AQFA04700 from Millipore and QF-200 from F&J Specialty Products) wereused to grow silicon nanowires. As described for growth on glass fibersubstrates, nanowires were grown on the quartz fiber filters at 480° C.for 90 minutes (FIG. 16 A-B). The silicon nanowires were removed fromthe filters by sonication (see, e.g., FIG. 16 C-D, which depict theresulting detached nanowires).

Growth of nanowires on porous or curved substrates such as meshes,microfibers, microbeads, or microporous glass or quartz materials offersa number of advantages over growth on planar substrates. For example,compared to a flat wafer surface, microfibers, microbeads, ormicroporous surfaces can grow nanowires with fewer collisions per wireat the same wire length and catalyst particle density due to the surfacecurvature of the substrate. Therefore, it is possible to obtain straightlonger wires free of branches at higher yield on curved or poroussubstrates than on flat surfaces. The total surface area of the microsubstrate can be easily controlled, for example, by varying the diameterand the volume of the micro-materials (e.g., the thickness and pore sizefor fiber membranes, or the diameter of beads). The high yield ofnanowires from the micro substrates can significantly reduce the cost ofmaking nanowires. Straight branch-free silicon nanowires can be producedusing a silane method or other un-oriented nanowire growth method onmicro substrates. If desired, the resulting nanowires can be easilyremoved by sonication after synthesis, due to the flexibility and/orsmall size of the substrate materials. In addition, when a poroussubstrate is employed, reacting gases can readily reach substantiallyall of the catalyst particles deposited on the substrate.

Simulations of nanowire growth provide an additional illustration of theadvantages offered by curved substrates as lattices for nanowire growth.FIG. 17A shows simulated randomly oriented 10 μm long nanowires growingon a 5 μm diameter fiber at a density of 0.5 nanowires/μm². Lightsquares mark collisions between wires. FIG. 17B graphs the number ofcollisions per nanowire as a function of the radius of the fiber, at twodifferent nanowire densities: 0.05 nanowires/m² and 0.5 nanowires/μm².As the radius of the fiber increases (and therefore, the curvature ofthe fiber's surface decreases), the number of collisions per wireapproaches that observed on a flat surface. As is evident from thegraph, the number of collisions per wire is affected by wire density andalso by the diameter of the fiber. A higher density of high-qualitynanowires (long and unbranched nanowires having few collisions per wire)can be obtained on a curved surface than a flat one. Furthermore, at aconstant density of nanowires (wires/area), the number of collisions perwire decreases with decreasing fiber radius.

Accordingly, the yield of long and/or unbranched nanofibers produced bythe methods is optionally greater than the yield of comparablenanofibers produced by synthesis on flat substrates. In one class ofembodiments, yield of the resulting nanofibers having a length greaterthan 10 μm (e.g., greater than 20 μm, 30 μm, 40 μm, 50 μm, or 60 μm) isat least 10% greater than yield of nanofibers of that length synthesizedon a planar non-porous substrate (i.e., a solid planar substrate with noapertures or pores therethrough) of the same surface area, usingsubstantially the same growth process. The yield from the methods isoptionally at least 25%, 50%, 75%, or even 100% greater than the yieldfrom growth on the planar non-porous substrate. For example, growth ofnanowires using a non-oriented synthesis technique, e.g., VLS growthfrom a gold colloid, can produce more long nanofibers on the substrateof a) or b) than on a flat non-porous substrate of comparable surfacearea using substantially the same growth process (e.g., the sametemperature, colloid deposition density, growth times, process gases,and the like).

In one class of embodiments, the curved substrate of b) has at least onedimension (typically, a cross-sectional diameter) that is less than1000, less than 500, less than 100, or less than 50 times an averagecross-sectional diameter of the nanofibers. The at least one dimensionof the substrate is optionally greater than 2, greater than 5, greaterthan 10, or greater than 20 times the average cross-sectional diameterof the nanofibers. The substrate optionally comprises a differentmaterial than the nanofibers.

As in the example above, the nanofibers are optionally removed from thesurface area of the substrate of a) or the curved surface of b)following synthesis of the nanofibers, e.g., by sonicating thesubstrate, to produce a population of detached nanofibers. As noted, themethod can produce long nanowires. Thus, in one class of embodiments, atleast 10% (e.g., at least 20%, 30%, 40%, 50%, 60%, 70%, 80%, or even90%) of the nanofibers in the population of detached nanofibers have alength greater than 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, or 60 μm, whileat most 50% (e.g., at most 40%, 30%, 20%, or 10%) of the nanofibers havea length less than 10 μm. Optionally, at least 50%, 60%, 70%, 80%, or90% of the nanofibers in the population of detached nanofibers areunbranched, while at most 50%, 40%, 30%, 20%, or 10% of the nanofibersare branched. The nanofibers can have similar length andbranched/unbranched distributions when attached to the substrate.

The methods optionally include characterizing the nanofibers, attachedto the substrate and/or after their removal from the substrate, bydetermining one or more of: their length, diameter, percentbranched/unbranched, collision with other nanowires per nanowire or perunit length of the nanowires, and the like, per nanowire or as anaverage or distribution.

Articles or populations of nanofibers produced by the methods formanother feature of the invention. Thus, one exemplary class ofembodiments provides an article comprising a substrate having a curvedsurface, and a plurality of nanofibers (e.g., nanowires) attached to atleast a portion of the curved surface of the substrate. The substratecan comprise, e.g., a plurality of microspheres or one or more glassfiber, quartz fiber, metallic fiber, or polymer fiber. An implantablemedical device comprising an article of the invention, e.g., attached toand covering at least a portion of the surface of the implantablemedical device, is also a feature of the invention.

All publications, patents, patent applications, and/or other documentscited in this application are incorporated by reference in theirentirety for all purposes to the same extent as if each individualpublication, patent, patent application, and/or other document wereindividually indicated to be incorporated by reference for all purposes.Although the present invention has been described in some detail by wayof illustration and example for purposes of clarity and understanding,it will be apparent that certain changes and modifications may bepracticed within the scope of the appended claims.

What is claimed is:
 1. A method of producing nanofibers, the methodcomprising: providing a plurality of microparticles or nanoparticleshaving a catalyst material deposited thereon; and synthesizing aplurality of nanofibers from the catalyst material on the microparticlesor nanoparticles.
 2. The method of claim 1, wherein the nanofiberscomprise nanowires.
 3. The method of claim 2, wherein synthesizing theplurality of nanowires comprises: depositing a gold colloid on at leasta portion of the overall surface area of the microparticles ornanoparticles; and growing the nanowires from the gold colloid with aVLS synthesis technique.
 4. The method of claim 1, wherein themicroparticles or nanoparticles comprise carbon black particles.
 5. Themethod of claim 1, wherein the microparticles or nanoparticles compriseglass or quartz microspheres.
 6. The method of claim 1, wherein theplurality of nanofibers comprises a semiconductor material selected fromgroup IV, group II-VI and group III-V semiconductors.
 7. The method ofclaim 1, wherein the plurality of nanofibers are comprised of silicon orsilicon oxide.
 8. The method of claim 1, comprising removing thenanofibers from the surface of the microparticles or nanoparticlesfollowing synthesis of the nanofibers, to produce a population ofdetached nanofibers.
 9. The method of claim 8, wherein said removingcomprises heating the microparticles or nanoparticles followingsynthesis of the nanofibers on the substrate to remove themicroparticles or nanoparticles from the nanofibers.
 10. The method ofclaim 9, wherein said heating comprises heating the microparticles ornanoparticles to a temperature greater than about 500 degrees Celsius.11. A composition comprising a population of carbon black particleshaving nanowires deposited thereon.
 12. The composition of claim 11,wherein the composition is conductive.
 13. The composition of claim 11,wherein the nanowires are comprised of silicon or silicon oxide.
 14. Aporous catalyst support comprising the composition of claim
 11. 15. Thecomposition of claim 11, further comprising one or more colloidparticles deposited on the carbon black particles.
 16. The compositionof claim 15, wherein the one or more catalyst particles comprise metal.17. The composition of claim 16, wherein the metal comprises gold orplatinum.